A*tron.  Dept. 


THE  ADOLFO  STAHL  LECTURES 
IN  ASTRONOMY 


THE  SOLAR  CORONA,  JUNE  8,  1918. 


From  a  photograph  with  the  40-foot  camera  of  the  Crocker  Eclipse 
Expedition  of  the  Lick  Observatory  to  Goldendale,  Washington. 
Exposure,  Om  21s  to  25s  of  totality.  South  is  at  the  left,  west  at  the 
top.  See  page  67,  footnote. 


THE  ADOLFO  STAHL  LECTURES 
IN  ASTRONOMY 


DELIVERED  IN  SAN  FRANCISCO,  CALIFORNIA,  IN 
1916-17  AND  1917-18,  UNDER  THE  AUSPICES  OF 
THE  ASTRONOMICAL  SOCIETY  OF  THE  PACIFIC 


FRANCISCO 


FOR  THE  SOCIETY 
THE  STANFORD  UNIVERSITY 


A7 

Astron,  Dop' 


COPYRIGHT,  1919,  BY 
THE  ASTRONOMICAL  SOCIETY  OF  THE  PACIFIC 


DEDICATION 


TO  MY  ILLUSTRIOUS  FRIEND,  MANUEL  ESTRADA 
CABRERA,  THESE  LECTURES  ARE  DEDICATED.  AS 
PRESIDENT  OF  THE  REPUBLIC  OF  GUATEMALA 
HIS  LOFTY  AIM  HAS  BEEN  TO  RAISE  THE  STAND- 
ARD OF  EDUCATION  AND  LITERARY  ACHIEVE- 
MENTS. HIS  ENERGY  HAS  BEEN  DEVOTED 
WHOLEHEARTEDLY  TO  ESTABLISHING  SCHOOLS 
OF  EVERY  DESCRIPTION  EVEN  IN  THE  REMOTEST 
PARTS  OF  HIS  COUNTRY;  LEARNING  OF  ANY 
TYPE  WHATSOEVER  HAS  AT  ALL  TIMES  HAD  HIS 
UNLIMITED  SUPPORT.  TO  WHAT  MORE  DESERVING 
PERSON,  THEREFORE,  COULD  ANY  LECTURES 
WHICH  HAVE  AS  THEIR  OBJECT  THE  DIFFUSION 
OF  KNOWLEDGE,  BE  PRESENTED? 


462946 


PREFACE 

From  the  time  of  its  organization  in  1889,  the  chief  object 
of  the  Astronomical  Society  of  the  Pacific  has  been  to  stimu- 
late interest  in  astronomy  among  the  people  of  the  Pacific 
region  by  giving  to  discoveries  and  advances  made  in  that  sci- 
ence the  widest  publicity  through  the  medium  of  its  Publica- 
tions and,  more  directly,  by  means  of  public  lectures.  In  har- 
mony with  this  policy,  plans  were  made  early  in  the  autumn  of 
1916  for  a  course  of  lectures  to  be  given  in  San  Francisco  by 
members  of  the  staff  of  the  Lick  Observatory,  in  the  season  of 
1916-1917. 

When  the  question  of  securing  financial  support  for  this 
undertaking  arose,  a  public-spirited  citizen  of  San  Francisco, 
Mr.  Adolfo  Stahl,  came  forward  and  offered  the  Astronomical 
Society  a  'sum  of  money  amply  sufficient  to  cover  all  the  neces- 
sary expenses  of  the  proposed  course.  Mr.  Stahl  imposed  no 
conditions  upon  his  gift  except  that  the  lectures  should  be  given 
in  San  Francisco,  that  they  should  be  adapted  to  the  under- 
standing of  all  intelligent  people,  and  that  they  should  later  be 
printed  in  the  Society's  Publications  or  elsewhere. 

Mr.  Stahl's  offer  was  gratefully  accepted  by  the  Society 
and  it  was  voted  that  the  course  of  lectures  be  known  as  THE 
ADOLFO  STAHL  LECTURES  IN  ASTRONOMY.  It  was  further 
voted  that,  in  recognition  of  this  gift,  Mr.  Stahl  be  elected  a 
patron  and  a  life  member  of  the  Society. 

The  lectures  were  given,  as  originally  planned,  by  Director 
W.  W.  Campbell  and_Astronomers  R.  G.  Aitken  and  H.  D. 
Curtis  of  the  Lick  Observatory,  in  Native  Sons'  Hall,  San 
Francisco,  in  November  and  December,  1916,  and  January, 
February,  March,  and  April,  1917. 

Since  it  was  clearly  Mr.  Stahl's  desire  that  the  lectures 
should  be  an  educational  influence  in  San  Francisco,  special 
efforts  were  made  to  direct  the  attention  of  the  teachers  and 
pupils  in  the  schools  of  the  city  to  the  opportunity  he  had  pro- 
vided for  securing  information  concerning  the  worlds  of  outer 
space.  That  the  opportunity  was  appreciated  was  evident  from 
the  large  attendance  at  all  six  of  the  lectures. 


Vlll 


PREFACE 


The  directors  of  the  Astronomical  Society  were  gratified 
by  the  interest  thus  shown  by  the  public  and  were  delighted 
when  Mr.  Stahl  most  generously  offered  to  repeat  his  gift,  to 
provide  for  a  second  series  of  lectures,  to  be  delivered  in  the 
season  1917-1918  under  the  same  conditions  as  the  first  course. 

The  committee  on  the  ADOLFO  STAHL  LECTURES  IN  ASTRON- 
OMY invited  members  of  the  staff  of  the  Solar  Observatory, 
Mount  Wilson,  and  of  the  staff  of  the  Students'  Observatory, 
Berkeley,  to  give  the  six  lectures  of  the  second  course.  The 
astronomers  of  these  two  institutions  cordially  accepted  the  in- 
vitations and  the  first  two  lectures  of  the  course  were  given  in 
Native  Sons'  Hall,  San  Francisco,  in  November  and  Decem- 
ber, 1916,  by  Professor  R.  T.  Crawford,  of  the  Students'  Ob- 
servatory, and  Astronomer  C.  E.  St.  John,  of  the  Solar  Observ- 
atory. The  sudden  and  serious  illness  of  one  of  the  other  lec- 
turers required  a  change  in  the  rest  of  the  program,  and,  to 
meet  the  emergency.  Astronomer  R.  G.  Aitken,  of  the  Lick 
Observatory,  gave  the  lecture  in  January,  1918.  The  lectures 
in  February,  March,  and  April,  1918,  were  delivered  by  Pro- 
fessor A.  O.  Leuschner,  Director  of  the  Students'  Observatory, 
and  Astronomers  F.  H.  Scares  and  G.  W.  Ritchey,  of  the  Solar 
Observatory. 

The  interest  shown  by  the  public  in  the  preceding  year  con- 
tinued, notwithstanding  the  general  unrest  created  by  the  entry 
of  our  country  into  the  world  war ;  and  this  interest  was  mani- 
fest not  only  at  the  times  when  the  lectures  were  delivered,  but 
also  in  the  numerous  requests  received  by  the  Society  that  the 
lectures  be  collected  and  published  in  book  form. 

These  requests  were  carefully  considered  by  the  directors 
of  the  Society,  and  the  chairman  of  the  Publication  Committee 
was  instructed  to  prepare  an  estimate  of  the  cost  of  publishing 
such  a  volume.  The  estimate  was  placed  before  Mr.  Stahl  in 
July,  1918,  and  he  was  pleased  to  stand  sponsor  for  the  Society 
in  the  matter. 

The  editorial  responsibility  for  the  volume  has  devolved 
upon  me,  but,  with  one  exception,  the  lectures  have  been  re- 
vised by  their  authors,  thus  lightening  the  burden.  It  is  a  pleas- 
ure to  acknowledge  here  the  indebtedness  of  the  editor  and  of 
the  Society  to  the  University  of  Chicago  Press,  the  Yale  Uni- 
versity Press,  the  Macmillan  Company,  Mrs.  Isaac  Roberts,  and 


PREFACE  ix 

the  directors  and  astronomers  of  the  Lick,  Lowell,  Mount  Wil- 
son, and  Yerkes  observatories  for  their  courtesy  in  supplying 
many  of  the  photographs  or  half-tone  blocks  used  in  the  illus- 
trations for  the  volume. 

It  is  the  editor's  privilege  to  express  in  this  place  the  high 
appreciation  in  which  Mr.  Adolfo  Stahl's  generous  gifts  are 
held,  not  only  by  the  directors  and  members  of  the  Astronomi- 
cal Society  of  the  Pacific,  but  also  by  all  those  who  found 
pleasure  in  the  lectures  his  generosity  provided.  We  recog- 
nize that  in  making  these  gifts  Mr.  Stahl  has  been  influenced 
primarily  by  the  desire  to  contribute  to  the  advancement  of  the 
intellectual  life  of  his  chosen  city;  but  we  trust  that  in  their 
present  form  the  STAHL  LECTURES  may  also  appeal  to  lovers  of 
astronomy  everywhere. 

R.  G.  AITKEN 


CONTENTS 

PAGE 

THE  SOLAR  SYSTEM       ...     .     7    .......     .     .     .     .       1 

W.  W.  CAMPBELL 

WHAT  WE  KNOW  ABOUT  COMETS  .........     26 

W.  W.  CAMPBELL 

A  TOTAL  ECLIPSE  OF  THE  SUN 52 

R.    G.    AlTKEN 

THE  MOON      .........     ^..  ......     76 

R.    G.    AlTKEN 

THE  NEBULAE      ,    .     .     .     .     .     .     .     .  -  .     .  ;'A  .'.     .     .     95 

H.  D.  CURTIS 

ASTRONOMICAL  DISCOVERY .     .     .     .     .110 

.       H.  D.  CURTIS 

THE   IMPORTANT    EPOCHS    IN    THE   DEVELOPMENT   OF 

ASTRONOMY     .     .     .     .     ;    i     .     ..   .     .     .     .     .     v   .  127 

R.  T.  CRAWFORD 

OUR  NEAREST  STAR,  THE  SUN 140 

C.  E.  ST.  JOHN 

NEWS  FROM  THE  STARS    .     .     .     ..     .......     .     .157 

Rr  G.    AlTKEN 

RECENT  PROGRESS  IN  THE  STUDY  OF  THE  MOTIONS  OF 

BODIES  IN  THE  SOLAR  SYSTEM       .     .     .     .     .'..-..  174 
A.  O.  LEUSCHNER 

THE  BRIGHTNESS  OF  THE  STARS,  THEIR  DISTRIBUTION, 

COLORS,  AND  MOTIONS     ........:..  208 

F.  H.  SEARES 

THE  100-INCH  REFLECTING  TELESCOPE,  MOUNT  WILSON  .  246 


LIST  OF  PLATES 

PLATE                                                                                                                                  FACING  PAGE 

Frontispiece    THE  SOLAR  CORONA,  JUNE  8,  1918 iii 

I.     JUPITER,  Photograph  by  E.  C.  Slipher 1 

II.     JUPITER,  Drazving  by  J.  E.  Keclcr 4 

III.  SATURN,  Drawing  by  J.  E.  Keclcr 10 

IV.  SATURN,    Photographs    by    E.    E.    Barnard;     MARS, 

Drawing  by  J.  E.  Kceler 15 

V.     MARS,    Drawings    by    Per  rival    Lowell    and    W.    H. 

Pickering    23 

VI.     HALLEY'S  COMET,  Photograph  by  H.  D.  Curtis 26 

VII.     DONATI'S  COMET,  Drawing;  HOLMES'S  COMET,  Photo- 
graph by  £.  E.  Barnard 33 

VIII.     LAG  OF  COMETS'  TAILS  ;  BREDICHIN'S  TYPES,  Diagram..  36 

IX.     RORDAME'S  COMET,  Photographs  by  W.  J.  Hnssey 40 

X.     BROOKS'S  COMET,  Photographs  by  E.  E,  Barnard. 43 

XI.     HALLEY'S  COMET,  Photographs  by  H.  D.  Curtis 46 

XII.     SPECTRA  OF  COMETS,* Photographs 49 

XIII.  THE  40-Foor  CAMERA,  FLINT  ISLAND 61 

XIV.  THREE  FLINT  ISLAND  VIEWS 66 

XV.     THE   INTRA-MERCURIAL   CAMERAS   AND  THE   MOVING- 
PLATE  SPECTROGRAPH,  FLINT  ISLAND 70 

XVI.     THE  MOON,  9  DAYS  OLD,  Photograph  by  E.  S.  Holden 

and  W.  W.  Campbell 76 

XVII.     THE  MOON,  19  DAYS  OLD,  Photograph  by  A.  L.  Colton 

and  C.  D.  Perrine 80 

XVIII.     THE  CRATER  ARCHIMEDES,  Photographs  by  E.  S.  Holden  86 

XIX.     THE  CRATER  PETAVIUS,  Photographs  by  E.  S.  Holden..  89 
XX.    THE  CRATER  COPERNICUS,  Drawing  by  L.  Wcinek,  based 

on  a  Lick  Observatory  Photograph 92 

XXI.     MESSIER  8;   N.  G.  C.  II.  5146;  DUMB-BELL  NEBULA, 

Photographs  by  H.  D.  Curtis •. 95 

XXII.     SPIRAL    NEBULAE,    Photographs    by    PI.    D.    Curtis; 

MESSIER  101 ;  Drawing  by  S.  Hunter 101 

XXIII.  SPIRAL  NEBULAE,  Photographs  by  H.  D.  Curtis 102 

XXIV.  NOVAE  IN  SPIRAL  NEBULAE,  Crossley  Reflector  Photo- 

graphs     107 

XXV.     THE  36-lNCH  REFRACTOR  OF  THE  LICK  OBSERVATORY 110 

XXVI.     THE  37-lNCH  MILLS  REFLECTOR,  SANTIAGO,  CHILE....  114 


XIV 


LIST  OF  PLATES 


PLATE  FACING  PAGE 

XXVII.     THE  72-lNCH   REFLECTOR,   DOMINION   ASTROPHYSICAL 

OBSERVATORY    118 

XXVIII.     B.   D.   CHART  AND   CROCKER  TELESCOPE  PHOTOGRAPH, 

M.  33  CENTRAL 120 

XXIX.     M.  33  TRIANGULI,  Photograph  by  J.  E.  Keeler 122 

XXX.    THE  MILLS  SPECTROGRAPH  ATTACHED  TO  THE  36-lNCH 

REFRACTOR    126 

XXXI.    THE  WELL  OF  ERATOSTHENES  ;  NEWTON'S  REFLECTOR....  128 

XXXII.     ISAAC  NEWTON 133 

XXXIII.  WILLIAM   HERSCHEL .' 134 

XXXIV.  WILLIAM   HUGGINS 136 

XXXV.     SIMON  NEWCOMB 138 

XXXVI.     SPECTRA  OF  THE  SUN,   SUN-SPOTS  AND  IRON   VAPOR, 

Mount  Wilson  Observatory  Photographs 143 

XXXVII.     PHOTOGRAPH    AND    SPECTROHELIOGRAM    OF    SUN-SPOT, 

Mount  Wilson  Observatory 145 

XXXVIII.     PROMINENCE  AT   LIMB   AND   ON   DISK   OF   THE    SUN, 

Photographs  by  F.  Ellerinan 148 

XXXIX.     COMBINED  PHOTOGRAPHS  OF  PROMINENCES  AND  FLOC- 

CULI,  Mount  Wilson  Observatory 151 

XL.     HYDROGEN  FLOCCULUS  DRAWN  INTO  SUN-SPOT,  Mount 

Wilson  Observatory  Photographs 154 

XLI.     THE  GREAT  NEBULA  IN  ORION,  Photograph  by  J.  E. 

Keeler  157 

XLIL    DARK  LANES  IN  TAURUS,  Photograph  by  E.  E.  Barnard     161 
XLIII.     CURVED  NEBULA  ABOVE  ORION,  Drawing  and  Photo- 
graph by  E.  E.  Barnard 164 

XLIV.    THE  PLEIADES,  Photograph  by  Isaac  Roberts 166 

XLV.     DARK  NEBULAE  IN  ORION  AND  IN  SAGITTARIUS,  Photo- 
graphs by  H.  D.  Curtis 168 

XLVI.     GREAT    NEBULA    IN    ANDROMEDA,     SHOWING     NOVAE, 

60-Inch  Reflector  Photograph 172 

XLVII.    LIGHTS  IN  VALLEY  BELOW  MOUNT  WILSON,  Photograph 

by  F.  Ell er man 208 

XLVIII.     KAPTEYN  "SELECTED  AREA"  No.  40,  60-Inch  Reflector 

Photograph 224 

XLIX.     REGION  OF  0  OPHIUCHI,  Photograph  by  E.  E.  Barnard..    232 

L.    THE  100-INCH  MIRROR 246 

LI.    TUBE-SECTION  OF  100-INCH  REFLECTOR  ON  THE  ROAD 

UP  MOUNT  WILSON 251 

LII.     MOUNTING  OF  THE  100-INCH  REFLECTOR 253 

LIII.     DOME  FOR  THE  100-INCH  REFLECTOR....  254 


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THE  SOLAR  SYSTEM1 

By  W.  W.  CAMPBELL 

The  study  of  astronomy  begins  naturally  with  the  solar 
system.  The  solar  system  is  our  abode.  It  is  the  observing 
station  from  which  we  look  out  in  all  directions  to  the  great 
stellar  system.  The  solar  system  is  only  a  minute  detail  in 
the  structure  of  the  universe.  He  who  would  explore  the 
universe  should  begin  by  knowing  his  immediate  surround- 
ings. Our  visual  telescopes  could  show  us  sixty  or  seventy 
millions  of  stars,  distributed  over  the  whole  sky,  and  our  great 
reflecting  telescopes  could  photograph  possibly  two  or  three 
times  as  many.  With  only  one  exception  all  of  these  stars  are 
so  far  away  that  they  are  seen  as  mere  points  of  light  in  our 
most  powerful  telescope,  even  when  the  magnification  is  nearly 
3,000  diameters.  The  one  exceptional  star  is  our  Sun.  It  alone 
of  all  the  stars  can  be  seen  to  have  a  "diameter."  It-  alone  of 
all  the  stars  can  be  studied  in  any  geometric  detail  by  means 
now  available.  This  is  because  our  Sun  is  relatively  near  to  us. 
The  next  nearest  star  known,  Alpha  Centauri,  is  275,000  times 
as  far  away  from  us  as  our  star  is.  If  we  would  know  what  the 
stars  in  general  are  we  should  begin  by  learning  about  our  own 
star.  That  is  the  chief  reason  why  there  are  solar  observatories 
in  many  countries  of  the  world.  Those  institutions  are  occupied 
wholly  or  chiefly  in  the  study  of  the  Sun.  Our  Sun  is  an 
exceedingly  interesting  body  in  itself,  especially  for  beings  who 
live  in  the  solar  system,  but  its  main  interest  to  astronomers 
lies  in  the  fact  that  knowledge  of  conditions  existing  in  our 
Sun  enables  us  to  draw  many  conclusions  concerning  conditions 
existing  in  millions  of  other  suns. 

It  is  not  our  purpose  to  describe  the  solar  system  in  detail , 
nor  shall  we  burden  the  lecture  by  quoting  the  enormous  dis- 
tances which  separate  the.  heavenly  bodies :  astronomers  do 
not  comprehend  them  any  better  than  the  laymen  do.  One  or 


Delivered  November   10,    1916. 


LECTURES 


two  distances,  one  or  two  masses,  will  be  sufficient  to  serve  as 
scale  values  for  the  entire  system.  We  shall  make  it  our  chief 
concern  to  emphasize  the  characteristic  features  of  the  solar 
system  so  that  we  may  comprehend  the  relation  of  its  different 
members  to  each  other,  and  the  relation  of  the  solar  system  as 
a  whole  to  the  great  stellar  system.  We  shall  try  to  visualize 
the  solar  system  as  it  exists  in  space,  highly  isolated  from  all 
other  members  of  the  stellar  system. 

The  solar  system  consists  of  the  great  central  Sun,  the 
eight2  major  planets  and  their  twenty-six  satellites,  the  more 
than  eight  hundred  minor  planets  or  asteroids,  the  zodiacal- 
light  materials,  the  comets  and  the  meteors.  Only  one  other 
class  of  bodies  is  known  to  astronomers:  the  nebulae.  Now 
many  of  the  nebulae  are  far  out  in  the  stellar  system,  and  a 
great  many  others  are  probably  outside  of  our  stellar  system. 
Certainly  none  of  the  nebulae  existing  today  have  a  direct 
connection  with  our  solar  system. 

We  have  said  that  the  Sun  is  one  of  the  ordinary  stars. 
Compared  with  the  thousands  of  other  stars  visible  to  the 
unassisted  eye  on  any  clear  night,  our  Sun  is  merely  an  average 
star.  Nevertheless  it  is  a  very  large  body.  Its  diameter  is 
110  times  the  Earth's  diameter.  Its  volume  is  therefore 
1,300,000  times  the  Earth's  volume.  If  the  Sun  were  a  hollow 
shell  of  its  present  diameter,  we  could  pour  more  than  a  million 
Earths  into  it  and  still  leave  empty  the  space  between  the 
Earth-balls. 

The  average  density  of  the  Earth  is  five  and  a  half  times 
that  of  water.  The  average  density  of  the  Sun  is  only  a 
quarter  that  of  the  Earth;  that  is,  the  Sun  is  forty  per  cent 
more  dense  than  water.  From  the  figures  quoted  it  follows 
that  the  mass  of  the  Sun,  in  other  words,  the  quantity  of 
material  that  the  Sun  contains,  is  333,000  times  that  of  the 
Earth.  It  is  this  immense  mass  which  gives  the  Sun  its  tre- 
mendous gravitational  power,  a  power  sufficient  to  maintain 
the"  planets  in  their  elliptic  orbits  around  it. 

At  an  average  distance  of  ninety-three  millions  of  miles 
from  the  Sun  are  the  Earth  and  its  Moon.  The  Earth-Moon 
system  revolves  once  around  the  Sun  in  what  we  have  agreed 

2  These   are   the   numbers   known  to   exist   in  the  year   1916. 


THE  SOLAR  SYSTEM  3 

to  call  our  year.  To  complete  the  circuit  in  the  year  requires 
the  Earth  to  travel  a  little  more  than  eighteen  miles  and  a  half 
per  second.  Between  the  Earth  and  the  Sun  are  two  known 
planets,  Mercury  and  Venus.  Mercury  is  a  little  planet, 
3,000  miles  in  diameter,  whose  average  distance  from  the  Sun 
is  about  three-eighths  the  Earth's  distance.  It  is  so  close  to 
the  Sun  that  it  must  travel  very  rapidly  in  its  orbit,  an  average 
of  twenty-eight  miles  per  second,  to  keep  from  being  drawn 
into  the  Sun.  Relatively  few  people  have  seen  Mercury.  It 
does  not  get  very  far  away  from  the  Sun,  but  if  observers 
know  when  it  is  going  to  be  at  its  greatest  distance  east  of  the 
Sun  shortly  after  sunset  and  west  of  the  Sun  shortly  before 
sunrise,  they  will  have  no  difficulty  in  seeing  the  planet  as  a 
first-magnitude  star  low  in  the  sky,  and  small  telescopes  will 
show  the  planet's  disk. 

The  planet  Venus,  whose  orbit  lies  between  those  of  Mer- 
cury and  the  Earth,  is  for  those  who  live  on  the  Earth  the  most 
brilliant  of  all  the  planets.  It  is  just  a  shade  smaller  than  the 
Earth  in  size.  The  Earth,  as  you  know,  is  a  little  over  7,900 
miles  in  diameter.  The  diameter  of  Venus  is  7,700  miles.  Its 
distance  from  the  Sun  is  not  quite  two-thirds  the  Earth's 
distance.  It  requires  seven  and  a  half  of  our  months  to  com- 
plete its  journey  around  the  Sun. 

Going  outward  from  the  Earth  we  come  to  our  interesting 
neighbor  Mars.  It  is  fifty  per  cent  farther  from  the  Sun  than 
we  are,  and  its  year  is  a  little  under  two  of  our  years.  Its 
diameter  is  slightly  more  than  one-half  the  Earth's  diameter — 
about  4,200  miles.  It  is  therefore  a  little  larger  than  Mercury 
and  a  good  deal  smaller  than  Venus  and  the  Earth.  It  has 
two  tiny  moons.  The  smaller  one  is  only  eight  or  ten  miles 
in  diameter,  and  the  larger  one  less  than  forty  miles.  The 
surface  areas  of  these  little  satellites  are  smaller  than  some  of 
the  counties  in  California. 

Next  in  order  of  distance  from  the  Sun  are  the  asteroids, 
or  little  planets.  The  first  one  was  discovered  on  the  first  day 
of  the  last  century,  and  up  to  the  present  time  more  than  800 
have  been  found.  It  is  not  an  uncommon  thing  for  ten  or  fifteen 
of  these  bodies  to  be  discovered  in  a  single  year,  by  means  of 
photography.  The  first  one  discovered  is,  so  far  as  we  know, 


4  THE  ADOLFO  STAHL  LECTURES 

the  largest:  a  little  less  than  500  miles  in  diameter.  The 
smallest  ones  are  certainly  less  than  ten  miles  in  diameter. 

The  largest  of  our  planets  is  Jupiter,  whose  average  distance 
from  the  Sun  is  a  little  over  five  times  the  Earth's  distance. 
Jupiter's  mean  diameter  is  eleven  times  the  Earth's.  Its 
volume  is  therefore  thirteen  hundred  times  the  Earth's  volume, 
and  if  that  planet  were  a  hollow  shell  you  could  pour  more 
than  one  thousand  Earths  into  it.  Jupiter  requires  nearly 
twelve  years  to  complete  its  circuit,  at  an  average  speed  of 
eight  miles  per  second.  This  great  planet  is  known  to  have 
nine  satellites.  The  four  bright  moons,  visible  even  with  opera 
glasses,  were  the  first  celestial  bodies  discovered  by  Galileo  and 
his  telescope — in  the  year  1610.  It  is  of  special  interest  to 
Californians  to  note  in/  passing  that  the  fifth  moon  of  Jupiter 
was  discovered  with  trie  36-inch  refractor  of  the  Lick  Observa- 
tory in  1892;  the  sixth  and  seventh  moons  with  the  Crossley 
reflector  of  the  Lick  Observatory  in  1904-5  ;  and  the  ninth 
satellite  with  the  Crossley  reflector  in  1914.  The  eighth  was 
discovered  at  the  Royal  Observatory,  Greenwich,  in  1908. 

Still  farther  from  the  Sun  is  Saturn  with  its  wonderful  ring 
system  and  nine  known  moons.  Its  mean  diameter  is  nine 
times  that  of  the  Earth,  and  it  goes  once  around  the  Sun  in  a 
little  less  than  thirty  years.  Maxwell  and  Keeler  proved  that 
the  rings  are  a  great  collection  of  little  moons — probably 
millions  of  them. 

The  six  major  planets  already  named  were  well  known  to 
the  ancients.  References  to  them  are  frequent  in  the  extant 
literature  of  the  nations,  past  and  present.  The  planet  next  in 
distance,  Uranus,  was  discovered  by  Sir  William  Herschel  in 
1781  in  one  of  his  famous  sweepings  of  the  heavens.  It  is 
nineteen  times  as  far  from  the  Sun  as  the  Earth,  its  diameter 
is  four  times  the  Earth's,  and,  traveling  four  miles  per  second, 
it  requires  eighty-four  years  to  complete  the  circuit  of  the 
Sun.  It  has  four  known  satellites. 

The  discovery  of  the  next  planet,  Neptune,  was,  as  you 
know,  a  great  event  in  the  history  of  astronomy.  Uranus  did 
not  follow  precisely  the  path  marked  out  for  it  by  astronomers, 
and  Adams  of  Cambridge  in  1845,  and  Leverrier  of  Paris 
independently  a  year  later,  proved  that  the  discrepancies  in  its 


PLATE  II.     DRAWINGS  OF  JUPITER  BY  JAMES  E.  KEELER. 

UPPER:     1889,  July  10d  10h  2m  P.  S.  T. 
LOWER:     1890,  Aug.  30d  llh  9m  P.  S.  T. 


THE  SOLAR  SYSTEM  5 

motion  could  be  caused  by  the  attractions  of  an  undiscovered 
planet  farther  from  the  Sun  than  itself.  They  computed  the 
position  of  the  undiscovered  planet.  Adams  tried  to  enlist  the 
services  of  the  greatest  telescopes  in  England  to  discover  the 
body,  but  his  advice  and  requests  were  neglected.  It  is 
especially  appropriate  in  these  days  of  war  between  the  nations 
to  note  this  illustration  of  the  international  character  of 
astronomical  research:  Leverrier  of  Paris  requested  Astrono- 
mer Galle  of  Berlin  to  search  for  the  new  planet,  with  the 
largest  telescope  on  the  continent.  Galle  found  the  planet  on 
the  first  night  of  the  search,  almost  exactly  where  Leverrier 
said  it  would  be.  Neptune  is  a  little  over  four  times  the  Earth 
in  diameter,  and  he  requires  165  years  to  travel  through  his 
orbit.  He  has  gone  less  than  half-way  around  the  Sun  since 
his  discovery.  Neptune  has  one  known  moon. 

It  is  not  impossible  that  other  planets  more  distant  than 
Neptune  are  revolving  around  the  Sun.  Several  astronomers 
have  devoted  much  time  to  searching  for  them. 

The  Earth-Moon  system  is  a  unique  combination,  in  that  the 
two  bodies  are  more  nearly  of  the  same  size  than  are  any  other 
planet  and  its  satellites.  The  Moon's  diameter  is  considerably 
over  a  fourth  of  the  Earth's  diameter.  It  required  the  Wash- 
ington 26-inch  telescope  to  discover  the  two  tiny  moons  of 
Mars,  but  an  astronomer  on  Mars  or  on  Venus,  when  those 
planets  are  in  favorable  positions,  would  not  need  any  tele- 
scope at  all  to  see  the  Earth  and  its  Moon  as  a  double  planet — 
the  only  double  planet,  so  to  speak,  in  the  solar  system. 

It  is  a  most  remarkable  fact  that  all  of  the  eight  major 
planets  and  all  of  the  more  than  800  asteroids  revolve  around 
the  Sun  in  the  same  direction,  which  astronomers  have  agreed 
to  call  from  west  to  east.  There  is  no  exception  to  this  rule. 

It  is  an  equally  remarkable  fact  that  the  eight  planets  re- 
volve in  orbits  lying  nearly  in  the  same  plane,  and  that  the 
average  position  of  the  orbital  planes  of  the  800  asteroids 
coincides  closely  with  the  average  for  the  eight  planets.  Let 
us  refer  the  planes  of  the  orbits  of  the  planets  and  asteroids  to 
what  we  may  call  the  average  plane  of  the  planets'  orbits. 
Mercury's  orbit  is  inclined  six  degrees  to  the  average  plane, 
and  Venus' s  orbit  two  degrees.  The  orbit  planes  of  the  other 


6  THE  ADOLFO  STAHL  LECTURES 

five  planets  are  inclined,  without  exception,  less  than  two 
degrees  to  the  average  plane  of  the  system.  The  orbit  planes 
of  a  few  of  the  little  asteroids  are  inclined  as  much  as  thirty 
or  forty  degrees  to  the  plane  of  the  system,  but  the  great  ma- 
jority of  the  asteroids  do  not  get  far  from  that  plane. 

Other  striking  and  related  facts  are  these :  The  Sun  rotates 
on  his  axis  from  west  to  east.  We  do  not  positively  know  the 
directions  of  rotation  for  Mercury  and  Venus,  but  there  are 
reasons  for  thinking  that  their  direction  is  also  from  west  to 
east.  Our  Moon  revolves  around  the  Earth  from  west  to 
east,  and  the  Earth  and  Moon  both  rotate  on  their  axes  from 
west  to  east.  Mars  rotates  from  west  to  east,  and  his  two 
moons  revolve  around  the  planet  from  west  to  east.  Jupiter 
and  Saturn  rotate  on  their  axes  from  west  to  east,  but  in  the 
satellite  systems  of  these  planets  and  in  the  systems  of  Uranus 
and  Neptune  we  come  upon  exceptions  to  the  west-to-east  rule. 
The  seven  inner  satellites  of  Jupiter  revolve  from  west  to  east, 
but  the  eighth  and  ninth  satellites,  which  are  farther  out  from 
the  planet  than  the  other  seven,  travel  from  east  to  west.  The 
eight  inner  satellites  of  Saturn  travel  from  west  to  east,  but 
the  far-out  ninth  reverses  the  direction.  The  four  moons  of 
Uranus  revolve  around  that  planet  in  a  plane  which  is  nearly 
at  right  angles  to  the  average  plane  of  the  planets,  and  the 
plane  of  the  satellites  of  Uranus  is  probably  the  approximate 
plane  of  the  equator  of  that  planet.  The  satellite  of  Neptune 
revolves  around  its  planet  from  east  to  west  in  a  plane  inclined 
at  an  angle  of  thirty-five  degrees  with  the  plane  of  the  planets. 
We  should  note  that  the  exceptional  cases  refer  to  the  outer- 
most planets  of  the  solar  system,  Uranus  and  Neptune,  and  to 
the  outermost  satellites  in  the  systems  of  Jupiter  and  Saturn. 
Everywhere  else  in  the  solar  system  prevails  the  rule  of  motions 
of  revolution  and  rotation  from  west  to  east. 

The  solar  system,  of  great  extent  in  the  plane  of  the  system, 
is  an  exceedingly  "thin"  system.  Let  us  call  the  distance  from 
the  Sun  to  the  Earth  one ;  then  the  distance  from  the  Sun  to 
the  outermost  planet,  Neptune,  on  the  same  scale  is  thirty,  and 
the  diameter  of  Neptune's  orbit  is  sixty.  Now  our  system  of 
Sun,  planets,  satellites  and  asteroids  lies  so  nearly  in  one  plane 
that  we  could  put  it  in  a  very  flat  bandbox,  sixty  units  in  diam- 


THE  SOLAR  SYSTEM  7 

eter  and  one  unit  in  thickness,  so  that  the  major  planets  and 
their  satellites,  and  all  the  asteroids,  with  a  very  few  exceptions, 
would  perform  their  motions  entirely  within  the  box.  The 
exceptional  asteroids  and  the  majority  of  the  comets  would  dip 
out  of  the  box  on  one  side  or  the  other  because  the  planes  of 
their  orbits  make  considerable  angles  with  the  central  plane  of 
the  solar  system. 

I  want  to  call  your  attention  as  forcibly  as  possible  to  the 
extreme  isolation  of  our  system  from  other  systems.  If  it  is 
one  unit  of  distance  from  the  Sun  to  the  Earth  and  thirty  units 
from  the  Sun  to  the  outermost  of  our  planets,  Neptune,  it  is,  on 
the  same  scale,  275,000  units  to  the  nearest  star  of  which  we 
have  any  knowledge,  Alpha  Centauri.  It  is  about  400,000  units 
in  an  entirely  different  direction  to  our  second  nearest  neighbor, 
and  so  on.  Most  of  the  comets  and  some  of  the  meteors,  as  we 
shall  learn  in  the  next  lecture,  travel  out  much  farther  from  the 
Sun  than  Neptune  is ;  but,  aside  from  some  of  the  comets  and 
meteors,  we  do  not  know  that  there  is  anything  in  space  between 
Neptune,  thirty  units  from  the  Sun,  and  the  nearest  star, 
several  hundred  thousand  units  from  the  Sun. 

Let  us  illustrate  our  isolation  in  still  another  way.  Light 
travels  from  the  Sun  to  the  Earth  in  eight  and  one-third 
minutes,  and  from  the  Sun  to  Neptune  in  four  and  a  half  hours ; 
but  it  requires  four  and  a  half  years  to  travel  from  our  Sun  to 
the  next  nearest  star,  Alpha  Centauri.  The  distance  of  Alpha 
Centauri  is  described  as  four  and  a  half  light-years.  The 
average  distances  between  the  stars  are  of  the  order  of  six  or 
seven  or  eight  light-years. 

It  must  be  clear  that  the  stars  and  the  planets  occupy  little 
space,  and  that  they  have  a  superabundance  of  room  to  move 
about.  We  have  found  that  the  average  speed  of  the  naked- 
eye  stars  in  their  motions  through  space  is  about  sixteen  miles 
per  second,  which  means  that  if  one  star  should  start  to  travel 
precisely  toward  its  nearest  neighbor,  assuming  its  nearest 
neighbor  to  be  at  the  average  distance,  it  would  require  some 
eighty  thousand  years  to  arrive  at  its  destination.  Now  the 
diameter  of  our  Sun,  an  average  star,  is  not  more  than  one 
fifty-millionth  as  great  as  the  average  distance  between  neigh- 
boring stars.  Under  such  conditions  it  is  not  difficult  to  see  that 


8  THE  ADOLFO  STAHL  LECTURES 

a  collision  of  two  stars  must  be  an  exceedingly  rare  event.  The 
approach  of  two  stars  so  close  as  to  disturb  each  other  violently 
must  also  be  rare.  However,  when  we  consider  the  number  of 
stars  in  the  stellar  system,  we  should  perhaps  expect  a  few 
close  approaches  to  occur  within  a  human  lifetime. 

The  researches  of  the  early  astronomers  were  confined 
almost  exclusively  to  the  solar  system.  Their  small  and 
imperfect  telescopes  lacked  the  power,  and  their  methods  lacked 
the  accuracy  for  attacking  the  problem  of  the  distant  stars. 
They  made  a  specialty  of  the  motions  of  the  bodies  which 
compose  the  solar  system,  of  their  forms  and  dimensions, 
and  of  their  orbits.  Their  labors,  supplemented  by  those  of 
astronomers  still  living,  have  been  so  thorough  and  complete 
that  we  can  predict  the  motions  of  the  planets  around  the 
Sun  and  the  motions  of  the  satellites  around  the  planets  with 
very  great  accuracy.  It  would  be  possible  to  compute  the  point 
in  the  sky  which  the  planet  Jupiter  will  occupy  one  hundred 
years  from  this  evening,  and  the  telescope  could  this  year  be 
directed  to  that  point  so  accurately  that,  on  looking  through  the 
telescope  one  hundred  years  from  tonight,  when  the  clock  said 
the  precise  second  had  arrived,  the  planet  would  be  seen  very 
close  to  the  center  of  the  telescopic  field  of  view.  The  eclipses 
of  the  Sun  are  computed  so  accurately  that  the  astronomer  may, 
if  he  chooses,  go  years  in  advance  to  the  proper  point  for 
observing  a  given  eclipse  and  direct  his  telescope  so  precisely 
to  the  position  which  the  eclipsed  Sun  will  occupy  as  to  witness 
the  phenomenon  when  it  arrives,  without  more  than  an  exceed- 
ingly small  change  in  the  pointing  of  his  instrument.  The 
pointing  of  the  instrument  would  probably  not  be  exactly  right, 
because  the  Moon  deviates  a  little  from  the  path  laid  down  for 
it ;  astronomers  do  not  know  why.  It  has,  in  fact,  been 
suggested  that  the  Moon's  motion  may  be  affected  slightly  by 
some  force  or  forces  whose  nature  has  not  yet  been  determined. 
There  is  likewise  an  appreciable  discrepancy  in  the  motion  of 
Mercury.  Whether  this  discrepancy  will  ever  be  removed  by 
virtue  of  a  more  complete  application  of  Newton's  law  of 
gravitation  to  the  problem  is  uncertain ;  some  other  force  than 
gravitation  may  be  acting,  but  it  need  be  only  a  very 
minute  force. 


THE  SOLAR  SYSTEM  9 

The  zodiacal  light,  a  faint  illumination  of  the  sky  visible 
above  the  Sun  when  the  Sun  is  a  few  degrees  below  the 
horizon,  is  an  interesting  phenomenon  surrounding  the  Sun. 
There  is  no  reason  to  doubt  that  the  zodiacal  light  which  we 
see  comes  originally  from  the  Sun,  and  that  this  light  falls 
upon  and  is  scattered  by  finely-divided  material — dust  grains  or 
very  small  bodies  in  great  numbers  which  revolve  around  the 
Sun,  each  such  particle  in  effect  a  little  planet.  This  material 
is  distributed  through  a  great  volume  of  space,  somewhat  in 
the  shape  of  a  double-convex  lens  whose  center  coincides  with 
the  Sun  and  whose  edge  extends  out  even  farther  than  the 
Earth's  orbit.  Its  shorter  dimension  extends  so  far  to  the 
north  and  to  the  south  of  the  Sun  that  northern  observers,  well 
situated,  may  see  the  zodiacal  light  at  midnight  in  May,  June 
and  July  above  the  northern  horizon. 

Belonging  to  the  solar  system  also  are  the  comets,  which 
pass  around  the  Sun  in  orbits  for  the  most  part  very  elongated. 
We  shall  study  the  comets  in  the  next  lecture. 

There  are  the  meteors,  many  of  which  revolve  around  the 
Sun  in  orbits  which  mark  them  as  members  of  the  Sun's 
system.  It  is  probable  that  some  of  the  meteors  are  merely 
passing  through  the  Sun's  system  and  are  not  of  it.  Occasionally 
a  meteorite  gets  down  through  our  atmosphere  to  the  Earth's 
surface,  is  found,  and  is  installed  in  a  museum ;  but  many 
millions  which  collide  with  our  atmosphere  every  twenty-four 
hours  are  consumed  by  the  friction  of  the  Earth's  atmosphere 
and  lose  their  identities. 

The  distribution  of  the  material  in  the  solar  system  is  most 
remarkable.  Nearly  all  of  it  is  in  the  Sun.  If  we  add  together 
the  masses  of  the  major  planets,  their  satellites,  the  hundreds  of 
asteroids,  make  liberal  allowance  for  the  masses  of  the -comets, 
meteors  and  zodiacal-light  materials,  and  call  the  total  one,  then 
the  mass  of  the  Sun  on  the  same  scale  is  744 ;  that  is,  of  745 
parts  of  matter  composing  our  solar  system  744  parts  are  in 
the  Sun  and  only  one  part  is  in  the  bodies  revolving  around 
it.  To  state  this  in  another  way :  ninety-nine  and  six-sevenths 
per  cent  (99%%)  of  the  material  of  the  solar  system  is  in 
the  Sun,  and  only  one-seventh  of  one  per  cent  (%%)  is 
divided  up  to  make  the  planets,  satellites,  asteroids,  etc.  The 


10  THE  ADOLFO  STAHL  LECTURES 

four  outer  planets,  Jupiter,  Saturn,  Uranus  and  Neptune, 
contain  225  times  as  much  material  as  the  four  inner  planets, 
Mercury,  Venus,  Earth  and  Mars.  The  Earth  is  fully  3,000 
times  as  massive  as  the  more  than  800  asteroids  com- 
bined. It  is  not  known  how  much  material  is  responsible 
for  the  zodiacal  light.  The  more  finely  divided  that  material  is, 
the  smaller  is  the  total  mass  required  to  reflect  and  scatter  the 
quantity  of  solar  light  observed  in  that  phenomenon.  Seeliger 
has  thought  that  the  scattered  zodiacal-light  materials,  if  con- 
densed into  one  body,  might  have  a  mass  fairly  comparable  to 
that  of  the  little  planet  Mercury,  and  he  has  concluded  that  the 
attractions  of  the  zodiacal-light  materials  upon  the  planet 
Mercury  could  explain  the  deviation  of  that  planet  from  its 
computed  orbit.  This  problem  cannot  yet  be  regarded  as 
definitely  settled.  For  several  decades  astronomers  thought 
there  might  exist  an  undiscovered  planet  or  planets  of  con- 
siderable size  between  the  Sun  and  the  orbit  of  Mercury  whose 
attractions  upon  Mercury  were  responsible  for  the  discrepancies 
in  its  motion.  The  work  of  the  Crocker  eclipse  expeditions 
from  the  University  of  California  is  morally  conclusive  that 
there  are  no  such  planets  massive  enough  to  explain  the 
observed  discrepancies.  We  do  not  know  the  mass  of  any 
single  comet,  but  we  do  know  that  cometary  masses  are  ex- 
ceedingly small  in  comparison  with  the  masses  of  the  smallest 
planets.  The  recent  comets  which  have  approached  close  to 
Mars,  Earth,  Mercury,  or  Venus  have  produced  no  appreciable 
disturbances  in  the  motions  of  those  planets. 

We  have  described  the  known  members  of  the  solar  system 
as  to  dimensions,  masses,  orbits  and  geometrical  relations  one 
to  another.  We  have  seen  that  they  form  the  Sun's  system — a 
system  very  completely  isolated  in  space,  and  independent  of 
other  systems  so  far  as  its  internal  relations  are  concerned.  Now 
the  solar  system  as  a  whole  is  traveling  through  space  with 
reference  to  the  other  members  of  the  stellar  system.  Sir 
William  Herschel  suggested,  a  century  and  a  third  ago,  that 
the  apparent  motions  of  the  other  stars  were  such  as  to 
indicate  a  motion  of  our  star  and  its  system  toward  the  con- 
stellation Hercules,  and  this  conclusion  has  been  amply  verified 
by  Herschel's  successors.  The  logic  of  the  demonstration  is 


THE  SOLAR  SYSTEM  11 

very  simple.  Let  us  use  an  illustration  which  every  one  has 
had  or  may  have  the  opportunity  to  test.  Suppose  the  observer 
is  traveling  rapidly  by  railway  train  across  a  level  tract  of 
country,  say  toward  the  west.  He  will  notice  that  the  trees, 
buildings,  or  other  objects  on  his  western  horizon  appear  to 
separate  gradually.  Similar  observations  on  the  trees  and 
other  objects  on  the  eastern  horizon  will  show  that  they  appear 
to  approach  each  other.  The  trees  and  buildings  on  the  horizon 
to  the  right  and  to  the  left  of  him  will  seem  to  be  traveling 
toward  the  east.  The  explanation  is  apparent.  The  motion  of 
the  solar  system  through  space  is  a  much  more  complicated 
problem,  in  that  we  must  deal  with  space  of  three  dimensions, 
instead  of  the  two  dimensions  of  the  terrestrial  surface,  and  the 
stellar  objects  which  the  observer  sees  in  all  directions  from  him 
are  themselves  in  motion.  However,  if  the  positions  of  a  great 
number  of  stars  have  been  accurately  determined  at  some  past 
epoch,  as  was  indeed  the  case,  and  the  recent  determinations  of 
positions  of  the  same  stars  be  compared  with  the  early  positions, 
it  will  be  found  that  the  stars  have  moved.  They  will  have 
moved  with  a  great  variety  of  speeds  in  a  great  variety  of 
directions;  but  if  the  stellar  motions  are  studied  with  care,  it 
will  be  found  that  the  prevailing  motion  of  any  great  group  of 
stars  in  any  large  area  of  the  sky  will  be  away  from  the  region 
of  the  constellation  Hercules  and  toward  the  opposite  point  of 
the  sky.  Herschel  reasoned  truly  that  this  prevailing  drift  of 
the  stars  away  from  the  constellation  Hercules  was  due  to  the 
motion  of  the  solar  system,  year  after  year,  decade  after  decade, 
toward  that  constellation.  Modern  solutions  of  the  same 
problem  have  changed  the  estimated  position  of  the  Sun's  goal 
very  slightly  toward  the  southeast,  to  a  point  near  the  boundary 
line  between  the  constellations  Hercules  and  Lyra. 

Astronomers  did  not  succeed  in  determining  the  speed  of  the 
solar  motion  from  these  apparent  motions  of  the  stars.  The 
difficulty  lay  in  the  fact  that  we  did  not  know  the  distances  of 
the  stars  whose  angular  motions  had  been  observed.  The  spec- 
trograph  has  enabled  the  second  part  of  the  problem  to  reach  a 
satisfactory  solution.  This  wonderful  instrument  enables  us  to 
measure  the  motions  of  approach  and  recession  of  the  stars,  and 
this  has  been  done  for  2,000  or  more  of  the  stars,  chiefly  under 


12  THE  ADOLFO  STAHL  LECTURES 

the  auspices  of  the  University  of  California,  by  the  Lick  Ob- 
servatory for  the  northern  stars,  and  by  the  D.  O.  Mills  Expe- 
dition to  Santiago,  Chile,  for  the  southern  stars.  It  has  been 
found  that  the  stars  have  a  great  variety  of  motions  of  approach 
and  recession.  If  we  examine  the  results  for  a  hundred  neigh- 
boring stars  in  some  one  large  area  of  the  sky,  we  shall  find 
that  a  few  will  be  approaching  the  solar  system  at  high  speed,  a 
few  will  be  receding  from  our  system  at  high  speed,  and  the 
others  will  be  represented  by  a  great  variety  of  motions  of  ap- 
proach and  recession.  This  happens  for  great  groups  of  stars 
in  any  part  of  the  sky.  If  we  consider  the  observed  motions  of 
100  stars  in  and  surrounding  the  constellations  Hercules  and 
Lyra,  we  shall  find  the  same  variety  of  speeds,  but  if  we  take 
the  average  speed  of  the  group  we  shall  find  that  the  group  as  a 
whole  seems  to  be  approaching  us  at  the  rate  of  about  twelve 
and  a  half  miles  per  second.  In  a  similar  manner,  if  we  con- 
sider the  motions  of  100  neighboring  stars  in  precisely  the  oppo- 
site region  of  the  sky,  we  shall  find  the  same  variety  of  approach 
and  recession,  but  we  shall  obtain  for  the  average  speed  of  the 
100  stars  as  a  group  an  apparent  recession  of  about  twelve  and 
a  half  miles  per  second.  No  one  questions  the  explanation  of 
these  observed  facts,  that  the  solar  system  is  traveling  toward 
the  Hercules-Lyra  region  with  a  speed  of  about  twelve  and  a 
half  miles  per  second  with  reference  to  the  system  of  naked-eye 
stars. 

Now  this  speed  of  motion  is  carrying  the  solar  system 
through  space  at  the  rate  of  approximately  400,000,000  miles 
per  year.  There  are  the  best  of  reasons  for  believing  that  our 
solar  system  is  very  old.  Its  age  can  scarcely  be  less  than  many 
tens  of  millions  of  years,  and  more  probably  hundreds  and 
thousands  of  millions.  It  is  clear  that  the  youth  of  the  solar 
system  was  spent  in  a  very  different  part  of  the  stellar  system 
from  where  it  now  is,  and  that  its  old  age  will  be  lived  in  a  still 
different  region.  We  do  not  know  whether  the  motion  of  the 
solar  system  follows  a  straight  line,  or  a  closed  curve  such  as  an 
ellipse,  but  the  system  is  probably  obeying  the  gravitational 
attraction  of  the  rest  of  the  material  universe.  It  seems  prob- 
able that  the  orbit  is  a  great  ellipse,  whose  circuit  is  so  great 
that  many  hundreds  of  miHions  of  years  will  be  required  to 


THE  SOLAR  SYSTEM  13 

travel  over  it  once,  even  though  our  system  meet  with  no  dis- 
turbing element  in  the  meantime. 

It  will  be  profitable  to  consider  briefly  the  conditions  existing 
in  the  Sun  and  planets.  Geologists  have  been  able  to  study  in 
a  limited  way  the  outcropping  geologic  strata  of  the  Earth,  but 
all  of  these  strata  combined  are  only  a  few  miles  in  thickness. 
There  are  indirect  ways  of  studying  the  interior  of  the  Earth, 
and  essentially  every  modern  student  of  the  subject  has  come  to 
the  conclusion  that  the  interior  of  the  Earth  is  solid  through- 
out, with  the  possible  exception  of  relatively  small  pockets  of 
molten  matter  here  and  there.  We  know  something  about  the 
oceans  and  the  atmosphere  of  the  Earth.  Do  any  of  the  other 
planets  resemble  the  Earth?  Mercury,  Venus  and  Mars  cer- 
tainly have  some  resemblances  to  our  planet,  but  the  giant 
planets  Jupiter,  Saturn,  Uranus  and  Neptune  are  extremely 
unlike  the  Earth.  The  Earth  appears  to  be  the  densest  of  all 
the  planets,  though  considerable  uncertainty  exists  as  to  the 
density  of  Mercury:  Venus  is  about  nine-tenths  as  dense  as  the 
Earth,  and  Mars  is  about  seven-tenths.  The  four  great  planets 
average  about  one-fifth  the  density  of  the  Earth.  Jupiter, 
Uranus  and  Neptune  are  a  little  more  dense  than  water, 
whereas  Saturn  is  so  light  that  if  it  could  be  thrown  upon  a 
great  terrestrial  ocean  it  would  float  like  a  piece  of  wood. 

We  can  get  no  trace  of  an  atmosphere  on  Mercury,  and  much 
remains  to  be  done  in  the  way  of  investigating  the  atmosphere 
of  Venus.  The  latter  planet  certainly  has  an  atmosphere,  but 
whether  it  is  comparable  in  quantity  and  chemical  composition 
with  the  Earth's  atmosphere  we  do  not  know.  As  Venus  is  only 
a  shade  smaller  than  the  Earth,  we  should  expect  the  at- 
mospheres of  the  two  planets  to  be  not  very  unequal.  We  know 
that  Mars  has  an  atmosphere,  but  it  is  a  very  light  one.  The 
Martian  atmosphere  at  the  surface  of  that  planet  is  probably 
not  over  one-half  the  density  of  the  Earth's  atmosphere  at  the 
summit  of  Mount  Everest,  our  highest  mountain  peak.  There  is 
no  reason  to  doubt  that  the  composition  of  the  Martian  atmos- 
phere is  very  much  like  our  own.  A  great  white  area  around 
the  north  pole  of  Mars  waxes  and  wanes  with  the  coming  and 
going  of  winter  in  the  northern  hemisphere  of  Mars,  and  a 
similar  white  cap  comes  and  goes  at  the  south  pole  of  the 


14  THE  ADOLFO  STAHL  LECTURES 

planet.  These  are  just  such  phenomena  as  occur  every  year  on 
the  Earth.  If  we  were  transported  a  few  thousand  miles  above 
the  northern  hemisphere  of  the  Earth,  we  should  see  a  great 
white  cap  growing  in  the  fall  and  winter  from  the  arctic  regions 
southward  across  Europe  and  Asia  to  the  latitudes  of  the 
Mediterranean  Sea  and  the  Himalaya  Mountains,  and  across 
Canada  and  the  United  States  well  toward  the  Gulf  of  Mexico ; 
and  we  should  see  the  southern  edge  of  this  cap  retreating 
northward  with  the  advent  of  spring  and  summer.  An  observer 
over  the  southern  hemisphere  of  the  Earth  would  witness  the 
annual  waxing  and  waning  of  the  white  cap  around  the  south 
pole  of  the  Earth,  save  as  the  southern  oceans  interrupted  its 
continuous  progress. 

The  four  giant  planets  have  enormously  extensive  atmos- 
pheres. We  appear  to  be  able  to  see  at  all  times  clouded  areas 
of  tremendous  extent.  These  clouds  are  more  prominent  in 
Jupiter  than  in  Saturn,  Uranus  and  Neptune,  but  that  the 
surfaces  of  all  four  have  a  very  high  percentage  of  clouds  we 
can  scarcely  doubt.  The  immense  masses  of  material  in  these 
planets  and  their  low  average  densities  lead  us  unavoidably 
to  conclude  that  they  are  not  solid,  as  in  the  case  of  the  Earth, 
but  that  they  are  largely,  and  perhaps  entirely,  in  a  gaseous 
state,  except  as  the  enormous  interior  pressures,  due  to  the 
overlying  strata,  may  liquefy  or  even  solidify  their  central 
volumes.  It  is  thought  that  the  gaseous  strata  in  each  of  the 
four  planets  extend  to  great  depths  and  that  there  is  nothing  in 
the  nature  of  a  solid  or  permanent  crust  over  the  surface  of  any 
of  them.  Their  low  densities  probably  mean  that  their 
enormously  deep  atmospheres  are  still  quite  hot.  Yet  we  have 
no  evidence  that  any  one  of  them  is  shining  by  its  own  light. 
When  one  of  Jupiter's  large  satellites  passes  between  the  Sun 
and  the  planet,  eclipsing  a  small  area  of  the  planet's  surface, 
that  area  looks  black,  but  this  may  be  in  part  a  contrast  effect. 

We  should  call  attention  to  the  flattened  forms  of  Jupiter  and 
Saturn.  The  rotation  of  the  Earth  once  in  about  twenty-four 
hours  has  caused  the  equatorial  regions  to  be  thrown  out  by 
centrifugal  force,  in  effect,  and  the  polar  regions  to  be  cor- 
respondingly drawn  in,  until  the  difference  between  the  equa- 
torial .and  polar  diameters  is  twenty-six  miles.  The  great 


C- 


FIG.  1— Photographs  of  Saturn,  Nov.  19,  1911,  60-inch  reflector  (100-foot 
focus)  of  the  Solar  Observatory.  Direct  enlargement  exposures  by 
E.  E.  Barnard. 


FIG.  2— Drawing  of  Mars,  May  29,  1890,  llh  45m  P.  S.  T.     36-inch  refrac- 
tor, Lick  Observatory,  by  J.  E.  Keeler. 


PLATE    IV. 


THE  SOLAR  SYSTEM  15 

planet  Jupiter  rotates  on  its  axis  in  a  little  less  than  ten  hours, 
whereas  the  little  Earth  takes  twenty-four  hours.  A  point  on 
Jupiter's  surface  is  traveling  by  rotation  some  twenty-seven 
times  as  rapidly  as  a  corresponding  point  on  the  Earth's  sur- 
face. The  centrifugal  force  is  enormous,  and  the  result  is 
easily  observable  in  the  equatorial  and  polar  diameters,  for 
there  is  a  difference  of  about  5,000  miles.  The  effect  is  even 
larger  in  the  case  of  Saturn,  where  the  difference  of  the 
diameters  is  nearly  7,000  miles.  The  throwing  of  the  clouds 
in  the  atmospheres  of  these  two  planets  into  belts  parallel  to 
the  equators  is  undoubtedly  connected  with  the  extremely  rapid 
rotations  of  the  planets,  probably  through  the  medium  of  trade 
winds  blowing  nearly  parallel  to  their  equators.  If  our  Earth 
rotated  more  and  more  rapidly  our  trade  winds  would  approach 
more  and  more  to  parallelism  with  the  equator. 

The  rings  of  Saturn  are  unique  in  the  solar  system.  Maxwell 
of  England  proved  by  mathematics,  and  Keeler  of  America 
proved  with  the  spectrograph,  that  these  rings  are  a  great 
collection  of  minute  and  separate  bodies.  There  are  so  many  of 
these  particles  or  separate  masses  that  they  seem  to  form  a  con- 
tinuous and  solid  system,  except  as  we  see  the  dark  lines 
dividing  them  into  several  component  rings.  If  the  rings  were 
solid  like  a  wagon  wheel,  to  use  a  homely  illustration,  the  outer 
edge  would  travel  by  rotation  more  rapidly  than  the  inner  edge. 
The  spectrograph  has  shown  that  the  reverse  is  the  case.  A 
moon  at  the  inner  edge  of  the  ring  system  would  have  to  travel 
very  rapidly  to  save  itself  from  falling  upon  the  planet.  A 
moon  at  the  outer  edge  of  the  ring  would  travel  much  more 
slowly.  Keeler  proved  that  each  point  of  the  ring  system  is 
traveling  with  the  speed  which  a  moon  at  that  distance  from 
the  center  of  the  planet  would  have.  Each  point  in  the  ring 
system  is  a  separate  moon  revolving  in  an  essentially  circular 
orbit  about  the  planet,  and  in  harmony  with  the  gravitational 
power  of  the  planet. 

Our  Moon,  as  you  know,  is  apparently  without  atmosphere 
and  water,  though  it  should  be  said  that  one  astronomer  thinks 
he  has  observed  changes  in  the  bottoms  of  the  lunar  craters, 
such  as  to  suggest  the  presence  of  a  trace  of  water  in  the  form 
of  frost  crvstals.  These  observations  should  be  verified  before 


16  THE  ADOLFO  STAHL  LECTURES 

they  are  interpreted  on  the  basis  of  water  vapor.     The  verifi- 
cation has  not  yet  been  provided. 

Most  interesting  of  all  the  bodies  in  the  solar  system  is  the 
Sun  itself.  It  is  an  intensely  hot  sphere  whose  outer  strata, 
certainly,  are  gaseous.  The  gaseous  composition  may  indeed 
extend  from  surface  to  center;  but  it  is  much  more  probable 
that  the  great  central  volume  is  in  the  liquid  or  even  solid 
state,  owing  to  the  tremendous  pressures  which  exist  there. 
We  know  that  the  surface  temperature  of  the  Sun  is  in  effect 
as  high  as  10,000°  Fahrenheit.  The  interior  temperatures  must 
be  vastly  higher.  The  chemical  elements  known  to  us  could 
exist  at  such  temperatures  only  in  the  form  of  incandescent 
gases  or  vapors,  except  as  immense  pressure  condenses  them 
to  the  liquid  or  solid  state.  We  know  that  our  atmosphere 
and  hydrogen  and  the  other  gaseous  elements  of  the  Earth  can 
be  liquefied  and  solidified  by  means  of  such  pressures  as  our 
laboratory  methods  are  able  to  produce.  The  pressures  in  the 
depths  of  the  Sun  run  up  into  the  millions  of  pounds  per  square 
inch;  and,  while  the  temperatures  there  existing  undoubtedly 
tend  to  preserve  the  gaseous  state  of  the  Sun's  interior,  the 
stupendous  pressure  probably  conquers  the  expansive  forces 
and  reduces  the  central  mass  to  the  liquid  or  solid  state.  It 
is  scarcely  possible  that  a  liquid  or  solid  core  extends  from  the 
center  out  to  near  the  surface  of  the  Sun,  for  the  average 
density  of  the  entire  body  is  only  1.4  times  the  density  of  water. 

About  forty  elements  familiar  to  us  on  the  Earth  have  been 
shown  to  exist  in  the  outer  strata  of  the  Sun  by  means  of  the 
spectroscope.  Rowland  has  said  that  if  the  Earth  were  heated 
up  until  its  temperature  was  equal  to  that  of  the  Sun,  the 
Earth's  spectrum  would  probably  resemble  closely  the  spectrum 
of  the  sun. 

When  we  look  at  the  Sun  we  see  what  we  call  the  photo- 
sphere. The  prevailing  opinion  of  the  photosphere  is  that  it 
consists  of  clouds  produced  by  the  condensation  of  some  of 
the  vapors,  formed  in  the  atmosphere  of  the  Sun  when  the 
conditions  for  condensation  are  right,  very  much  as  our  own 
clouds  form  in  our  atmosphere  when  the  conditions  are  right. 
The  clouds  of  water  vapor  with  which  we  are  familiar  form 
at  a  lo.w  temperature  because  we  are  dealing  with  water  which 


THE  SOLAR  SYSTEM  17 

has  a  freezing  temperature  of  -\-32°  Fahrenheit.'  Clouds 
would  be  expected  to  form  from  iron  vapor  at  a  very  high 
temperature,  for  the  freezing  point  of  iron  is  about  1500° 
above  zero  Fahrenheit. 

The  atmosphere  of  the  Sun  is  in  rapid  circulation.  There 
are  great  storms  in  its  atmosphere,  vastly  more  violent  than 
those  in  the  Earth's  atmosphere.  In  terrestrial  storms  there 
are  great  whirling  disturbances  in  our  atmosphere.  The  sun- 
spots  are  to  us  the  outward  and  visible  sign  of  somewhat 
similar  storms,  on  a  tremendous  scale.  The  motions  of  gases 
and  vapors  in  sun-spots  have  been  measured  by  means  of  the 
spectroscope,  and  Hale  has  shown  that  sun-spots  are  the  centers 
of  local  magnetic  fields.  The  magnetic  field  is  probably 
developed  in  each  case  by  the  rapid  rotation  of  electrically 
charged  particles  within  the  volume  of  spot  disturbance. 

The  sun-spots,  as  well  as  other  details  of  the  Sun's  surface, 
reveal  a  curious  law  of  solar  rotation.  The  entire  Sun  is 
rotating  rapidly  from  west  to  east,  but  the  equatorial  regions 
are  rotating  more  rapidly  than  the  regions  of  high  latitude. 
Areas  near  the  equator  rotate  once  around  in  twenty-four  days, 
but  at  forty-five  degrees  of  north  and  south  latitude  the  rotation 
period  is  twenty-eight  days,  and  at  seventy-five  degrees  of 
north  and  south  latitude  the  period  is  thirty-three  days.  The 
forging  ahead  of  the  equatorial  regions  has  never  been 
satisfactorily  explained. 

The  sun-spots  vary  in  size  most  curiously,  and  for  reasons 
unknown.  The  spottedness  passes  from  minimum  to  maximum 
and  back  again  to  minimum  in  an  average  period  of  eleven  and 
one-tenth  years.  During  the  years  of  minimum  it  is  not  unusual 
for  the  spots  to  be  entirely  absent  for  weeks  at  a  time.  The 
curve  which  represents  the  spottedness  of  the  Sun  as  observed 
from  the  year  1740  up  to  1870  shows  that  twelve  maxima 
and  twelve  minima  occurred  in  this  interval.  The  maxima 
and  minima  do  not  come  with  perfect  regularity.  Some- 
times a  maximum  is  a  year  or  two  early,  or  a  year  or  two  late, 
and  similarly  for  the  minima.  Many  investigators  have  tried 
to  find  an  explanation  of  the  sun-spot  period,  but  the  results 
have  not  been  satisfactory.  The  cause  has  been  looked  for 
in  the  action  of  the  planets.  It  would  seem  that  if  any  of  the 


18  THE  ADOLFO  STAHL  LECTURES 

planets  is  responsible  it  should  be  the  giant  Jupiter.  There  is 
no  apparent  connection,  however,  for  Jupiter's  period  about  the 
Sun  is  11.9  years,  whereas  the  sun-spot  period  is  11.1  years. 
It  has  been  suggested  that  the  spots  are  formed  when  two  or 
more  of  our  planets  are  in  the  same  straight  line  with  the 
Sun,  but  the  fact  is  there  are  just  as  many  spots  visible  when 
the  planets  are  equably  distributed  around  the  Sun,  with  no 
two  of  them  in  or  near  a  straight  line  with  the  Sun.  The  cause 
of  periodicity  seems  to  lie  within  the  Sun  itself.  It  is  perhaps 
not  impossible  that  certain  forces  develop  and  accumulate 
within  the  Sun  until  they  reach  the  breaking-out  intensity,  once 
in  eleven  years,  somewhat  after  the  fashion  of  the  forces  which 
are  responsible  for  the  geysers  on  the  Earth.  I  do  not  mean  to 
convey  the  impression  that  the  motions  within  sun-spots  and 
the  motions  of  water  expelled  from  geysers  are  the  same,  as 
they  are  not. 

Experienced  investigators  have  tried  to  find  a  relationship 
between  sun-spots  and  terrestrial  weather,  but  they  have 
not  succeeded  in  proving  that  such  is  the  case.  There  have 
been  and  still  are  people  who  say  that  the  sun-spots  rule  our 
weather,  but  they  seem  not  to  know  what  constitutes  a  scien- 
tific proof;  at  least,  no  proof  has  been  published.  They  re- 
mind me  of  the  small  boy's  first  experience  with  an  electric 
trolley  car.  The  street-cars  in  his  town  had  been  drawn  by 
horses,  and  he  had  no  doubt  as  to  the  motive  power.  There 
came  a  morning  when  he  and  his  father  got  on  a  successor  to 
the  horse-car,  and  he  was  interested  to  know  what  made  the 
car  go.  His  father  tried  to  explain  that  it  was  electricity,  but 
the  boy  was  not  convinced,  and  this  is  not  surprising,  for  nobody 
even  now  knows  what  electricity  is.  Before  he  got  to  the  end 
of  his  trolley  ride  he  said,  "Father,  I  have  discovered  what 
makes  this  car  go.  It  is  that  bell  up  there  above  the  driver's 
head,  for  I  have  noticed  that  every  time  that  bell  rings  the  car 
starts."  Therefore,  according  to  the  same  logic,  as  there  are 
spots  on  the  Sun  and  there  are  rain  storms  on  the  Earth,  the 
sun-spots  cause  the  rain.  Unfortunately  it  happens,  now  and 
then,  that  we  have  an  exceedingly  dry  winter  month  when  the 
Sun  is  rich  in  spots,  and  a  wet  month  has  been  prophesied ;  and 
that  we  have  an  exceedingly  wet  winter  month,  now  and  then, 
when  no  spots  whatever  are  visible.  It  rains  no  more  in  the 


THE  SOLAR  SYSTEM  19 

three  or  four  years  of  sun-spot  maximum  than  it  does  in  the 
three  or  four  years  of  sun-spot  minimum.  Likewise,  the 
storms  are  no  more  numerous  and  no  more  severe  when  there 
are  two  or  three  planets  almost  exactly  in  line  with  the  Sun 
than  when  the  planets  are  equably  distributed  around  the  Sun. 

In  one  respect  we  are  sure  that  the  sun-spots  do  have  a 
terrestrial  influence.  Magnetic  disturbances  on  the  Earth  are 
directly  related,  in  some  way,  to  the  sun-spot  activity.  The 
curve  of  magnetic  disturbances  when  correlated  with  the  curve 
of  solar  spottedness,  shows  an  agreement  that  is  unmistakable. 

Outside  and  beyond  the  spherical  body  of  the  Sun  which  we 
see  every  clear  day  are  the  prominences  and  the  corona.  The 
prominences  are  certainly  connected  with,  or  are  the  fruits  of, 
the  circulatory  system  of  the  Sun's  atmosphere.  They  require 
special  spectroscopic  apparatus  for  their  observation  in  ordi- 
nary times,  but  they  can  be  seen  directly  at  times  of  solar 
eclipses,  when  the  main  body  of  the  Sun  is  hidden  behind  the 
Moon  and  the  background  of  sky  is  relatively  dark.  They  are 
of  great  variety  as  to  forms  and  speeds  of  development.  They 
sometimes  shoot  up  to  heights  of  two  or  three  hundred  thousand 
miles  above  the  Sun's  surface,  with  speeds  as  high  as  250 
miles  per  second. 

The  solar  corona  may  also  be  a  product  of  the  rapid  circu- 
lation within  the  Sun's  structure.  It  is  not  impossible  that  the 
materials  composing  the  corona  are  expelled  from  the  Sun  by 
something  in  the  nature  of  volcanic  force,  or  by  the  pressure  of 
the  intense  solar  rays  upon  the  minute  particles  of  the  corona, 
or  by  other  force  or  forces,  and  that  these  particles  find  their 
way  back  in  descending  streams  to  the  Sun.  The  corona  is  a 
part  of  the  Sun.  A  complete  understanding  of  our  Sun  re- 
quires a  study  of  the  corona,  and  it  is  chiefly  for  investigations 
of  this  solar  appendage  that  eclipse  expeditions  are  dispatched 
to  the  out-of-the-way  corners  of  the  Earth.  It  has  been  found 
that  the  form  of  the  corona  depends  upon  the  spottedness  of 
the  Sun.  At  times  of  spot  maximum  the  corona  is  nearly  cir- 
cular in  general  outline,  whereas  at  times  of  minimum  the 
coronal  streamers  which  extend  out  from  regions  of  low  lati- 
tude are  extremely  long,  and  the  streamers  which  originate  at 


20  THE  ADOLFO  STAHL  LECTURES 

high  latitudes  and  in  the  vicinity  of  the  poles  of  the  Sun  are 
very  short.8 

People  in  general  know  that  the  Sun  is  vital  to  life  on  the 
Earth,  but  they  do  not  realize  that  all  other  sources  of  energy 
are  negligible.  The  Sun's  light  and  heat  grow  the  farmers' 
crops.  The  solar  radiation  grows  the  forests  of  today.  It  grew, 
long  ages  ago,  the  luxuriant  vegetation  which,  submerged  and 
compressed,  is  the  coal  that  today  drives  the  railway  trains  of 
the  land  and  the  ships  of  the  sea.  It  is  the  Sun's  power  which 
evaporates  the  water  of  the  ocean  and  creates  the  winds  which 
carry  the  evaporated  water  over  the  mountains  where  it  is 
deposited  as  rain  and  snow.  Our  hydro-electric  plants  control 
the  descent  of  this  water  from  the  mountains  to  the  sea,  and 
their  water-wheels  and  dynamos  generate  electric  current.  The 
Sun's  energy  thus  transformed  illuminates  our  cities  and  drives 
the  trolley  cars.  We  do  not  depend  at  all  upon  the  Earth's 
internal  heat.  The  temperature  of  the  Earth's  surface  is 
determined  by  the  heat  received  from  the  Sun.  To  realize  this 
fact,  let  us  recall  the  frigid  conditions  perpetually  existing  at 
the  poles  of  the  Earth.  During  several  weeks  in  the  middle  of 
the  northern  summer  the  north  pole  receives  more  solar  heat 
than  any  other  region  of  the  Earth,  and  throughout  the  year 
some  of  the  heat  from  the  tropics  and  the  north  temperate  zone 
is  constantly  transmitted  by  atmospheric  circulation  to  the 
north  polar  region.  Similarly,  during  several  weeks  in  the 
middle  of  the  southern  summer  the  south  pole  of  the  Earth 
receives  more  heat  than  any  other  region  of  the  Earth,  and 
constantly  throughout  the  year  some  of  the  heat  of  the  tropics 
and  of  the  south  temperate  zone  is  conveyed  through  the 
atmosphere  to  the  region  of  the  south  pole.  Yet  how  frigid 
and  essentially  useless  in  the  vegetable  and  animal  world  are 
the  polar  regions !  The  interior  heat  of  the  Earth  is  not  able  to 
do  anything  appreciable  for  those  regions.  Now  if  the  Sun's 
heat  were  cut  off  completely  from  the  Earth  for  one  short 
month,  the  equatorial  regions  would  be  at  the  end  of  the  month 
so  wintry  that  the  north  and  south  polar  regions  as  they  are 
today  are  rose  gardens  in  comparison. 


a  The   observed   form   of   the   corona  on   June   8,    1918,   seems   to   call    for   some 
modification    of   this    hypothesis. 


THE  SOLAR  SYSTEM  21 

To  create  due  respect  in  our  minds  for  the  overwhelming 
power  of  the  Sun,  we  may  reflect  upon  the  following  statement : 
When  the  Sun  is  directly  or  approximately  over  any  region  of 
the  Earth  and  our  atmosphere  above  that  region  is  in  normally 
clear  condition,  each  square  yard  of  that  region  receives  energy 
from  the  Sun's  rays  at  the  approximate  rate  of  four-fifths  of 
one  horsepower.  This  is  at  the  rate  of  4,000  horsepower  per 
acre.  If  you  own  250  acres  of  desert  in  Arizona,  or  Mexico, 
or  northern  Africa,  the  Sun  in  the  middle  of  each  summer  day 
is  pouring  down  energy  upon  your  little  ranch  at  the  rate  of 
one  million  horsepower.  Your  neighbor's  ranch  of  the  same 
size  is  receiving  solar  energy  at  the  same  rate.  And  so  on  for 
the  entire  surface  of  the  Earth,  in  proportion  as  the  Sun's  rays 
fall  perpendicularly  or  slantingly  upon  each  area.  Yet  this  is 
far  from  the  whole  story.  Nearly  the  half  of  the  energy  which 
the  Sun  tries  to  send  to  the  Earth's  surface  is  intercepted  by 
our  atmosphere  and  turned  back  into  space.  With  the  Sun 
directly  overhead  for  the  various  regions  of  the  Earth,  only 
about  sixty  per  cent  of  the  Sun's  energy  gets  down  through  the 
atmosphere  to  the  land  and  water  surface  of  the  Earth,  and  the 
remainder  is  refused  transmission.  Now  the  Sun,  to  the  best 
of  our  knowledge,  is  sending  out  energy  in  all  directions  at 
essentially  the  same  rate.  The  little  Earth  covers  so  small  an 
area  of  the  sky,  as  one  would  see  the  sky  if  he  were  on  the  Sun, 
that  the  Earth  intercepts  only  one  two-billionth  part  of  the 
Sun's  radiation.  If  we  could  cover  the  Sun  with  a  shell  of  ice 
forty  feet  thick,  the  heat  energy  radiated  from  the  Sun,  at  its 
present  rate,  would  be  sufficient  to  melt  that  shell  of  ice  in  one 
minute  of  time.  To  produce  this  quantity  of  energy  from  the 
combustion  of  coal  would  require  that  a  layer  of  the  best 
anthracite  twelve  or  fifteen  feet  deep  over  the  entire  solar 
surface  be  consumed  every  hour.  Now,  if  the  Sun  were 
composed  of  anthracite  the  consumption  of  the  whole  mass 
would  not  furnish  sufficient  heat  to  supply  the  Sun's  output,  at 
the  present  rate,  for  as  long  as  10,000  years. 

It  was  Kant  in  the  eighteenth  century,  and  Helmholtz  inde- 
pendently a  hundred  years  later,  who  showed  that  the  contrac- 
tion of  the  Sun  under  the  influence  of  its  own  gravitational 
power  is  the  most  probable  explanation  of  its  source  of  heat; 


22  THE  ADOLFO  STAHL  LECTURES 

perhaps  not  its  sole  source,  but  a  source  which  would  suffice 
to  maintain  the  present  rate  of  radiation  for  many  millions  of 
years.  The  Sun's  own  gravitational  power  is  struggling  con- 
stantly to  draw  every  one  of  its  particles  to  the  center  of  the 
Sun;  it  is  subjected  to  its  own  immense  compressive  force. 
Now  we  know  that  when  we  compress  air,  for  the  purposes 
of  industry  or  to  fill  an  automobile  tire,  a  great  quantity  of 
heat  in  the  air  compressed  is  liberated  and  radiated  into  sur- 
rounding space.  In  the  same  way  the  constant  and  stupendous 
process  of  compression  which  the  Sun  suffers  from  its  own 
internal  gravitation  liberates  the  heat  that  is  latent  within  its 
mass.  The  immense  quantity  of  energy  represented  by  the 
actual  motion  of  the  Sun's  materials  inward  toward  the  cen- 
ter is  also  converted  into  heat.  These  are  such  fruitful  sources 
of  heat  that  the  Sun  need  contract  no  more  than  300  feet  per 
year  at  the  present  time  to  supply  the  radiation  which  goes 
out  in  all  directions  and  of  which  a  very  little  reaches  us  upon 
the  Earth.  This  is  so  slow  a  rate  of  solar  contraction  that 
we  could  not  hope  to  observe  any  diminution  in  the  Sun's 
diameter,  even  with  our  most  refined  measuring  apparatus, 
until  after  the  passing  of  some  5,000  years.  There  can  be  no 
doubt  that  this  solar  compression  will  liberate  sufficient  heat 
to  maintain  the  present  rate  of  flow  for  five  or  ten  millions 
of  years,  and  it  can  be  shown  by  the  application  of  the  same 
principles  that  the  Sun  may  well  have  been  radiating  heat  at 
an  approximately  equal  rate  for  five  or  ten  millions  of  years 
in  "the  past.  It  is  essentially  certain  that  the  radium  within 
the  Earth  is  a  powerful  factor  in  developing  the  Earth's  in- 
ternal heat.  We  have  no  evidence  as  to  the  existence  of 
radium  in  the  Sun,  but  it  or  some  of  its  radio-active  relations 
may  be  there  to  assist  in  giving  long  life  to  the  Sun  and  to  the 
planets  which  draw  their  sustenance  from  the  Sun. 

What  can  be  said  as  to  the  existence  of  life  on  the  other 
bodies  of  the  solar  system?  We  may  dismiss  the  Sun  as  too 
hot  to  support  any  form  of  life  with  which  we  are  acquainted. 
Our  Moon  cannot  support  life,  at  least  of  the  terrestrial  kinds, 
because  of  the  total  lack  of  air  ar!<^  water.  The  probabilities 
are  strong  that  Mercury  is  lifeless,  for  the  same  reason,  but  this 
is  not.  a  certainty.  I  think  we  may  dismiss  Jupiter,  Saturn, 


N 


1 


FIG.  1.  FIG.  2. 

FIGS.  1  AND  2.     By  Percival  Lowell,  in  Mars  and  Its  Canals,  pp.  126,  229. 


FIG.  3.  FIG.  4. 

FIGS.  3  AND  4.    By  William  H.  Pickering,  in  Popular  Astronomy,  Jan.,  1918. 

PLATE  V.     DRAWINGS  OF  MARS. 


THE  SOLAR  SYSTEM  23 

Uranus  and  Neptune  as  abodes  of  life :  we  do  not  see  how  they 
can  have  anything  in  the  nature  of  solid  surfaces.  Venus  and 
Mars  are  the  planets  most  nearly  equal  in  size  to  the  Earth. 
Mars  has  a  very  light  atmosphere,  certainly,  but  we  know 
nothing  as  to  the  extent  of  Venus' s  atmosphere,  except  that  it 
has  one.  If  Schiaparelli  was  right  in  his  conclusion  that  the 
planet  Venus  always  presents, the  same  face  to  the  Sun,  as  it 
probably  does,  then  life  on  Venus  would  be  difficult :  one  hemi- 
sphere would  have  eternal  day  with  burning  temperatures,  and 
the  other  hemisphere  eternal  night  with  extreme  cold.  Mars 
and  the  Earth  seem  to  have  many  resemblances.  Seasonal 
changes  occur  in  the  aspect  of  Mars  such  as  could  reasonably  be 
attributed  to  changes  in  vegetation  ;  and  if  there  is  vegetable  life 
there  could  well  be,  and  probably  is,  animal  life.  However,  the 
vegetable  may  be  easily  independent  of  the  animal ;  the  forests 
and  prairies  of  the  Mississippi  Valley  put  on  their  green  cloth- 
ing in  the  spring  of  every  year  and  changed  to  brown  clothing 
in  the  fall  of  every  year  even  better  before  the  coming  of  "in- 
telligent" man  than  after  his  appearance  on  the  scene.  The 
"canals"  of  Mars  may  be  evidence  of  intelligent  life  on  that 
planet ;  but  unless  we  accompany  them  with  some  rather  violent 
assumptions  the  canals  could  serve  equally  well  as  examples  of 
the  lack  of  intelligence  on  the  planet.  How  would  engineers  on 
the  Earth  proceed  to  catch  the  water  from  the  melting  north 
polar  cap  of  the  Earth  and  use  it  for  irrigation,  not  only  south 
to  the  equator,  but  well  down  into  the  southern  hemisphere? 
How  would  they  reverse  the  process  and  use  the  waters  from 
the  south  polar  cap  to  irrigate  not  only  as  far  north  as  the  equa- 
tor but  well  into  the  northern  hemisphere — it  being  assumed 
that  there  are  no  oceans  to  interfere?  Would  intelligent  en- 
gineers insist  on  running  their  canals  absolutely  straight  for 
thousands  of  miles,  or  would  they  follow  the  contours  ?  As  the 
surfaces  of  the  Earth  and  the  Moon  are  exceedingly  nnlevel,  is 
it  reasonable  to  assume  that  Mars,  half-way  between  the  Earth 
and  the  Moon  in  size,  has  a  level  surface?  Mars  probably  has 
animal  life,  but  in  my  opinion  we  have  not  the  proof  of  it. 

I  think  it  is  impossible  for  an  intelligent  and  thoughtful 
mind  to  contemplate  the  orderly  solar  system,  completely 
isolated  from  other  systems,  its  great  Sun  in  the  center,  the 


24  THE  ADOLFO  STAHL  LECTURES 

tiny  planets  and  the  infinitesimal  asteroids  revolving  around  the 
Sun  in  the  same  direction  and  nearly  in  a  common  plane,  the 
moons  revolving  .around  the  planets,  all  of  the  planets  and 
asteroids  around  the  Sun  from  west  to  east,  and  nearly  all  of 
their  moons  around  their  planets  from  west  to  east,  without 
saying  to  ourselves :  the  members  of  the  solar  system  have  had 
a  common  origin ;  the  materials  in  the  Sun,  the  planets  and 
moons  have  had  a  prior  existence  under  other  conditions ;  and 
the  operation  of  the  laws  of  nature  has  developed  the  system 
to  its  present  state,  and  will  guide  its  further  development  to 
the  state  which  the  future  has  in  store  for  it.  Kant's  hypothesis 
would  have  the  development  proceed  from  a  great  collection 
of  matter  in  a  chaotic  state — the  same  matter  which,  trans- 
formed and  redistributed,  now  composes  the  system.  Laplace's 
hypothesis  would  develop  the  solar  system  from  a  rotating 
parent  nebula.  Chamberlin  would  have  the  antecedent  nebula 
spiral  in  structure.  This  phase  of  the  subject  would  demand  a 
full  hour  for  adequate  treatment,  and  we  must  be  content  to  say 
that  all  astronomers  believe  the  solar  system  to  be  the  product 
of  evolution. 

Are  there  other  solar  systems  than  ours  ?  Are  there  planets 
revolving  around  the  other  stars,  as  our  planets  revolve  around 
our  Sun  ?  Is  there  life  on  planets  in  other  systems  ?  We  do  not 
know.  We  are  powerless  to  answer  these  questions  at  present. 
If  we  should  transport  our  astronomers  and  their  most  power- 
ful instruments  to  Alpha  Ccntauri,  the  solar  system's  nearest 
neighbor,  they  could  not  look  back  and  see  the  planets  which 
attend  our  Sun.  They  would  see  our  Sun  by  naked  eye  as  a 
first-magnitude  star,  but  our  greatest  planet,  Jupiter,  would  be 
a  star  of  the  twenty-first  magnitude,  and  their  telescopes  at 
Alpha  Centauri  would  have  to  be  at  least  twenty-five  feet  in 
diameter  in  order  to  show  Jupiter  as  a  stellar  point  of  light,  just 
on  the  limit  of  vision,  even  though  the  flood  of  light  from  our 
Sun  did  not  interfere  with  the  observation.  The  fact  is  that 
Jupiter,  as  seen  from  Alpha  Centauri,  would  never  be  more 
than  five  seconds  of  arc  from  our  Sun,  and  the  glare  of  sunlight 
in  the  Centauran  telescope  would  hopelessly  drown  the  image 
of  Jupiter,  even  though  the  diameter  of  the  telescope  were  much 
greater  than  twenty-five  feet.  The  latter  difficulty  would 


THE  SOLAR  SYSTEM  25 

resemble  that  of  trying  to  see  a  glow  worm  that  is  two  feet  to 
the  right  or  left  of  a  powerful  searchlight  located  sixteen  miles 
from  the  observer. 

Although  we  have  not  been  able  to  secure  direct  and  positive 
evidence  in  favor  of  other  planetary  systems,  and  although  we 
see  no  promise  of  such  evidence  in  the  future,  it  would  be 
unreasonable  to  believe  that  such  planetary  systems  do  not 
exist.  It  would  be  contrary  to  the  simple  probabilities  if  our 
Sun,  one  of  several  hundred  millions  of  suns,  were  the  only 
Sun  attended  by  planets,  and  our  Earth  were  the  only  planet 
that  was  the  abode  of  life.  We  are  not  able  to  prove  that  we 
have  neighbors  scattered  throughout  the  great  stellar  universe, 
but  we  are  justified,  I  think,  in  believing  that  they  are  there. 


WHAT  WE  KNOW  ABOUT  COMETS1 

By  W.  W.  CAMPBELL 

The  startlingly  sudden  appearance  of  some  great  comets, 
the  rapid  growth  of  others  to  enormous  sizes  and  their  equally 
rapid  disappearance  have  naturally  excited  the  interest  and, 
only  too  often,  the  fears  of  the  human  race.  We  are  removed 
less  than  two  centuries  from  the  long-prevailing  theological 
view  that  comets  are  flaming  fire-balls  hurled  at  the  Earth  by 
an  angry  God,  to  frighten  and  punish  a  sinful  world.  Up  to 
the  time  of  my  childhood  the  opinion  was  widespread  among 
civilized  peoples  that  comets  are  the  forerunners  of  famine, 
pestilence  and  war.  Did  not  the  great  comet  of  1811  herald 
the  war  of  1812;  the  comet  of  1843  the  war  of  1846;  and 
Donati's  comet  of  1858  our  Civil  War?  Even  in  the  twentieth 
century  the  fear  that  a  comet  may  collide  with  the  Earth  and 
destroy  its  inhabitants  comes  to  the  surface,  here  and  there, 
every  time  a  comet  is  visible  to  the  naked  eye.  This  fear  is 
not  lessened  by  the  highly  sensational  descriptions  of  such 
encounters  by  professional  writers  who  have  that  little  knowl- 
edge which  has  been  called  a  dangerous  thing. 

The  Earth  has  undoubtedly  encountered  comets'  tails  scores 
and  scores  of  times  since  the  advent  of  man,  and  with  no  bane- 
ful effects;  and  in  the  light  of  present-day  knowledge  of  the 
structure  and  chemical  composition  of  comets  there  is  no  danger 
whatever  that  our  atmosphere  will  be  poisoned  by  such  an 
encounter.  It  is  true  that  a  collision  between  the  Earth  and 
the  head  of  a  comet  could  happen,  but  we  see  no  reason  to 
question  the  accuracy  of  the  estimates  made  by  mathematical 
astronomers  that  such  encounters  will  not  occur  more  than 
once  in  fifteen  or  twenty  million  years,  on  the  average !  It  is  by 
no  means  certain  that  such  an  encounter,  should  one  ever 
occur,  would  be  a  serious  matter  for  the  Earth.  Its  effects 
might  be  confined  to  a  brilliant  shower  of  meteors,  such  as  the 
peoples  of  the  Earth  have  observed  many  times.  Geologists 


Delivered   December   8,    1916. 


PLATE  VI.     HALLEY'S  COMET,  MAY  1,  1910;  HEAD  AND  BEGINNING 

OF  TAIL. 


Photograph  by  H.  D.  Curtis. 


WHAT  WE  KNOW  ABOUT  COMETS  27 

are  of  the  opinion  that  the  outcropping  strata  of  the  Earth 
which  they  have  been  able  to  study  have  required  a  period  of 
approximately  one  hundred  million  years  for  their  formation. 
These  strata,  embracing  the  entire  land  area  of  the  Earth,  have 
given  only  one  bit  of  evidence  that  the  Earth's  surface  has 
been  affected  by  a  collision  with  an  outside  body.  In  central 
Arizona  is  a  cup-shaped  hole  in  the  ground,  about  three 
quarters  of  a  mile  in  diameter  and  several  hundred  feet  deep, 
which  has  been  formed,  with  little  doubt,  by  the  descent  of  a 
great  meteorite,  or  of  a  great  cluster  of  small  meteorites : 
thousands  of  small  iron  meteorites  have  been  found  in  and  all 
around  the  hole,  and  there  are  no  evidences  of  volcanic  activity 
in  the  crater  and  its  immediate  surroundings.  Geologic  and 
geographic  surveys  of  the  Earth  have  revealed  no  other  case 
of  collisional  effects2  in  the  records  of  a  hundred  million  years. 
Man  himself  has  lived  upon  the  Earth  certainly  many  tens  of 
thousands  of  years,  and  there  are  no  traditions  extant  concern- 
ing injuries  to  earth  or  to  man  from  comets.  Why  then  should 
anybody  worry  about  possible  injury  from  a  comet  in  his  short 
span  of  three-score  years  and  ten? 

The  answer  to  our  first  question,  where  do  comets  come 
from,  involves  the  question  of  their  relationship  to  the  solar 
system  and  to  the  great  stellar  system.  It  is  essential  that  every 
auditor  should  understand  certain  prominent  features  of  the 
solar  and  stellar  systems ;  and,  at  the  risk  of  repeating  what 
many  members  of  the  audience  already  know,  I  shall  devote  a 
few  lines  to  a  description  of  these  systems. 

Widely  scattered  throughout  a  great,  but  finite,  volume  of 
space  occupied  by  our  stellar  system  are  tens  of  millions  of 
stars.  It  is  estimated  that  our  largest  refracting  telescopes 
could  show  us  about  seventy  million  stars,  and  that  the  reflect- 
ing telescopes  could  photograph  possibly  two  or  three  times  as 
many.  Our  own  Sun  is  just  one  of  these  scores  of  millions  of 
stars.  It  seems  very  large,  very  bright  and  very  hot  because 
we  on  the  Earth  are  relatively  close  to  it.  It  is  our  own  star. 
Revolving  around  it  are  many  planets,  of  which  our  Earth  is 
one.  Probably  the  other  stars  in  many  cases,  possibly  in  all 
cases,  have  planets  revolving  around  them  in  the  same  way. 

2  Neglecting  the   insignificant  cavities  produced  by  isolated  small   meteorites. 


28  THE  ADOLFO  STAHL  LECTURES 

We  do  not  know  that  this  is  a  fact  because  the  nearest  star, 
excepting  our  own  star,  is  so  far  away  that  we  should  require 
telescopes  at  least  twenty-five  feet  in  diameter  to  see  planets 
revolving  about  it,  even  though  such  planets  be  as  large  as 
Jupiter  and  Saturn,  the  largest  planets  revolving  around  the 
Sun. 

Now  the  Sun  and  its  planets  and  their  moons  are  the  chief 
members  of  an  orderly  system  which  we  call  the  solar  system. 
Ninety-nine  and  six-sevenths  per  cent  of  all  the  materials  in 
the  solar  system  is  in  the  Sun,  and  only  one-seventh  of  one  per 
cent  is  divided  up  to  form  the  planets  and  their  moons : 
Mercury,  Venus,  the  Earth  and  its  one  moon,  Mars  and  its  two 
moons,  the  more  than  eight  hundred  minor  planets  which  move 
in  the  zone  lying  just  outside  of  the  orbit  of  Mars,  the  giant 
planet  Jupiter  and  its  nine  moons,  the  planet  Saturn  with  its 
ring  system  and  its  nine  moons,  the  planet  Uranus  and  its  four 
moons,  and  the  outermost  known  planet  Neptune  and  its  one 
moon. 

It  is  a  most  interesting  fact  that  all  of  these  planets  revolve 
around  the  Sun  in  the  same  direction,  which  astronomers  have 
agreed  to  call  from  west  to  east,  or  in  the  "direct"  sense. 
Motion  from  east  to  west  is  called  "retrograde". 

Another  remarkable  fact  is  this :  the  orbits  of  all  these 
bodies  lie  nearly  in  the  same  plane.  If  we  call  the  distance 
from  the  Sun  to  the  Earth  unity,  then  the  distance  from  the 
Sun  to  the  outermost  planet,  Neptune,  on  the  same  scale  is 
thirty  units,  and  the  diameter  of  the  solar  system  on  that  scale 
is  sixty  units.  If  we  had  a  great  box  sixty  such  units  in  diam- 
eter  and  only  one  unit  in  thickness  the  solar  system  could  be 
placed  within  this  box  and  all  of  the  eight  major  planets  and 
their  moons  and  nearly  all  of  the  minor  planets  would  perform 
their  motions  within  the  box.  A  few  of  the  minor  planets 
would  dip  a  little  out  of  the  box,  above  or  below. 

The  solar  system  is  very  completely  isolated  in  space.  If 
the  distance  from  the  Sun  to  the  Earth  is  one  and  from  the 
Sun  to  Neptune  thirty,  then  the  distance  to  the  next  nearest 
star  of  which  we  have  any  knowledge,  Alpha  Centauri,  is 
275,000.  A  ray  of  light  traveling  with  a  speed  of  186,000 
miles  .per  second  would  travel  from  the  Sun  to  the  Earth  in 


WHAT  WE  KNOW  ABOUT  COMETS 


29 


eight  and  one-third  minutes,  to  Neptune  in  four  and  a  half 
hours,  but  it  would  require  four  and  a  half  years  to  reach  the 
Sun's  nearest  neighbor,  Alpha  Centauri.  The  stars  in  the 
great  stellar  system  are  distributed  more  or  less  irregularly, 
but  their  average  distance  apart  is  of  the  order  of  six  or  seven 
or  eight  light-years. 

All  of  the  stars  are  in  motion,  and  our  own  star,  the  Sun, 
is  no  exception  to  the  rule.  It  is  one  of  the  well-established 
facts  of  astronomy  that  our  solar  system  is  traveling  through 
space  in  the  general  direction  of  the  boundary  line  between  the 
constellations  Lyra  and  Hercules  with  a  speed  of  approxi- 
mately twelve  and  one-half  miles  per  second. 


Mars  \ 


FIG.  1.    CHARACTERISTIC  FORMS  OF  ORBITS. 

It  is  well  known  that  the  orbits  of  our  planets  are  ellipses 
which  do  not  differ  greatly  from  the  circular  form.  The 
comets,  on  the  other  hand,  move  in  very  elongated  orbits 
around  the  Sun.  The  orbits  of  some  comets  are  easily 
recognized  as  ellipses,  but  for  the  great  majority  of  comets  the 
orbits  differ  but  little  from  the  parabolic  form.  The  parabola, 
as  many  of  you  know,  is  on  the  dividing  line  between  ellipses 
and  hyperbolas.  The  ellipse  is  a  closed  curve,  and  a  comet 
moving  around  the  Sun  in  an  elliptic  orbit  should  return  again 


30  THE  ADOLFO  STAHL  LECTURES 

and  again  to  the  neighborhood  of  the  Sun;  but  a  comet 
following  a  parabolic  or  hyperbolic  path,  subject  merely  to  the 
attraction  of  the  Sun,  can  pass  through  the  vicinity  of  the 
Sun  only  once,  for  the  parabola  and  the  hyperbola  are  not 
closed  curves,  and  the  branch  upon  which  the  comet  approaches 
the  Sun  and  the  branch  upon  which  the  comet  recedes  from  the 
Sun  never  come  together,  no  matter  how  far  out  from  the 
Sun  they  be  drawn. 

There  have  been  two  hypotheses  as  to  where  the  comets 
come  from.  Sir  Isaac  Newton  thought  of  them  as  moving  in 
elongated  ellipses.  It  was  the  view  of  Immanuel  Kant  160 
years  ago  that  comets  are  bona  fide  members  of  the  solar  sys- 
tem, just  as  the  Earth  and  Neptune  are:  that  their  orbits  are  all 
ellipses,  but  very  elongated  ellipses.  He  said  that  the  comets 
travel  out  a  great  distance  from  the  Sun,  but  that  they  must 
eventually  return  because  they  are  moving  in  ellipses.  Kant's 
view  of  the  subject  was  essentially  a  mere  opinion,  though  the 
opinion  of  one  of  the  greatest  philosophers  of  all  time,  who 
gave  careful  consideration  to  every  known  fact.  Up  to  Kant's 
day,  and  for  many  decades  later,  comet  observations  were 
crude  in  comparison  with  present-day  standards.  Most  comets 
were  observed  for  only  a  few  weeks,  and  the  true  characters 
of  the  very  elongated  orbits  could  not  be  affirmed. 

Half  a  century  later  the  great  Laplace  championed  the  view 
that  the  comets  belong  to  the  stellar  system  and  not  to  the  solar 
system;  that  comets  are  travelers  through  interstellar  space; 
that  the  wanderings  of  a  chance  few  comets  bring  them 
within  the  sphere  of  influence  of  our  Sun;  and  that  we  see 
those  which  come  into  favorable  position  near  the  Earth. 
Halley's  celebrated  comet  was  the  only  one  then  known  to 
return  again  and  again  to  the  region  of  the  Sun,  and  it  was 
thought  to  be  a  captured  wanderer.  In  Laplace's  time  also  the 
comets  were  still  inaccurately  observed,  over  short  periods  of 
time,  and  in  nearly  every  case  a  parabola  seemed  to  represent 
their  motion  satisfactorily.  This  Laplacean  view  that  comets 
are  wanderers  through  the  great  stellar  system  and  are  only 
chance  visitors  to  the  solar  system  was  the  prevailing  one 
throughout  the  nineteenth  century.  Evidences  to  the  contrary 
began- to  appear  as  early  as  1860,  but  so  firmly  rooted  was  the 


WHAT  WE  KNOW  ABOUT  COMETS  31 

hypothesis,  that  only  in  the  twentieth  century  have  astronomers 
in  general  been  convinced  that  the  comets  are  members  of  the 
solar  system.  Several  lines  of  evidence,  all  in  good  agree- 
ment, have  brought  us  to  this  conclusion. 

1.  Since  the  solar  system  is  traveling  through  the  stellar 
system  in  the  direction  of  the  constellations  Lyra  and  Hercules, 
with  a  speed  of  twelve  and  a  half  miles  per  second,  if  comets 
come  in  from  interstellar  space  we  should  meet  more  comets 
coming  from  the  Lyra-Hercules  direction  than  there  are  comets 
overtaking  us  from  the  opposite  part  of  the  sky,  for  precisely 
the  same  reason  that  if  we  are  traveling  very  rapidly  by 
automobile  from  San  Diego  to  Los  Angeles  we  should  meet 
more  autos  than  would  overtake  us  and  pass  us.  Now  the 
comets  do  not  show  that  preference.  As  early  as  1860  Car- 
rington  studied  the  directions  of  approach  of  all  the  comets, 
133  in  number,  which  up  to  that  time  were  considered  to  have 
parabolic  or  hyperbolic  orbits.  He  found  that  only  sixty-one3 
of  these  comets  met  the  solar  system,  so  to  speak,  whereas 
seventy-two3  comets  overtook  us — extremely  strong  evidence 
that  the  comets  are  traveling  along  with  us,  just  as  all  of  our 
planets  are  traveling  with  the  Sun  while  revolving  around  it. 
Many  later  astronomers,  especially  Fabry,  using  the  more 
plentiful  and  more  accurate  data  now  available,  have  confirmed 
this  conclusion  that  there  is  no  tendency  for  comets  to  meet 
us,  as  we  rush  through  interstellar  space,  rather  than  to  over- 
take us.  It  is  a  fact,  however,  that  the  observed  comets  have 
not  had  their  directions  of  approach  distributed  uniformly  over 
the  surface  of  the  sphere.  Their  deviations  from  reasonable 
uniformity  appear  to  be  due  in  small  measure  to  a  preference 
of  comets  to  travel  in  planes  making  small  angles  with  the 
ecliptic,  with  motion  around  the  Sun  from  west  to  east  as  in 
the  case  of  the  planets;  but  the  chief  discrepancies  arise  from 
the  heterogeneous  circumstances  under  which  comets  are  dis- 
covered. 

Nearly  all  discoveries  of  comets  made  by  means  of  tele- 
scopes prior  to  forty  years  ago  were  made  in  the  northern 
hemisphere,  at  observatories  situated  in  latitudes  north  of 
+40°.  The  southern  hemisphere  is  still  very  much  in  arrears 

;!  The  disparity  in  the  numbers  is  thought  to  be  purely  accidental. 


32  THE  ADOLFO  STAHL  LECTURES 

in  the  matter  of  comet  discoveries,  though  the  discrepancy  is 
not  now  so  great  as  it  once  was. 

There  is  more  searching  for  comets  in  the  northern  hemi- 
sphere during  the  northern  summer  and  in  the  southern  hemi- 
sphere during  the  southern  summer  than  in  their  respective 
winters.  There  is  also  a  better  chance  for  northern  observers 
to  discover  comets  when  the  Sun  is  farthest  north  in  June  and 
for  southern  observers  when  the  Sun  is  farthest  south  in 
December.  These  facts  lead  to  the  discovery  of  comets, 
prevailingly,  which  come  to  perihelion  in  certain  favored 
regions ;  that  is,  in  the  regions  of  the  sky  where  the  Earth  is 
at  those  times. 

It  is  advantageous  at  this  point  to  call  attention  to  other 
sources  of  lack  of  homogeneity  in  comet  data. 

Prior  to  the  invention  of  the  telescope,  three  centuries  ago. 
about  400  comets  had  been  made  matters  of  historical  record. 
These  were  naked-eye  objects  which  forced  themselves  upon 
the  attention  of  observers.  They  were  the  especially  large 
comets  which  came  close  to  the  Earth  or  to  the  Sun.  They 
were  imperfectly  observed,  and  for  only  a  small  proportion  of 
them  do  we  know  even  their  approximate  orbits. 

Since  the  invention  of  the  telescope,  about  450  comets  have 
been  discovered,  and  the  half  of  these  have  been  found  in  the 
last  fifty  years.  What  we  may  call  the  golden  age  of  comet 
discovery  included  the  two  decades,  1888  to  1908,  when  100 
comets,  an  average  of  five  per  year,  were  discovered.  Four 
American  observers,  Swift,  Brooks,  Barnard  and  Perrine, 
announced  the  arrival  of  thirty-seven  of  these  100  comets. 

All  of  the  early  comets  were  visible  to  the  naked  eye.  Only 
a  small  fraction  of  recent  comets,  perhaps  one  in  four,  become 
bright  enough  for  the  unassisted  eye  to  see  the  head,  and 
perhaps  one  in  eight  or  ten  for  the  unassisted  eye  to  see  the 
tail.  Computed  comet  orbits  have  become  increasingly  accu- 
rate, partly  because  of  greater  telescopes,  which  enable  these 
bodies  to  be  more  accurately  observed  and  observed  through 
longer  arcs  of  their  orbits. 

2.  Another  decisive  argument  for  the  theory  that  comets 
are  at  home  in  the  solar  system  is  this :  Schiaparelli  showed 
in  the. early  70's  that,  owing  to  the  Sun's  motion  through  the 


FIG.  1 — Donati's  comet,  1858,  Oct.  5 ;  head  and  beginning  of  tail ;  brilliant 
stellar  nucleus  near  center  of  head;  envelopes  surrounding  nucleus 
on  side  toward  the  Sun.  White  circle  to  the  left  represents  com- 
parative size  of  the  Earth. 


FIG.  2— Holmes's  comet  of  1892 ;  no  tail  was  visible  in  the  telescope ;  long- 
exposure  photographs  (Barnard,  3  hours,  Nov.  10,  1892)  recorded 
an  extremely  faint  tail  extending  down  to  lower  right  corner  of  the 
picture.  The  great  spiral  nebula  in  Andromeda  was  recorded  on 
the  photograph — upper  left  corner  of  picture. 

PLATE  VII. 


WHAT  WE  KNOW  ABOUT  COMETS  33 

stellar  system,  if  the  comets  come  from  distant  interstellar 
space,  a  very  large  proportion  of  them  should  move  around 
our  Sun  in  hyperbolic  orbits,  and  many  of  these  orbits  should 
be  strongly  hyperbolic.  Schiaparelli's  conclusions  have  been 
confirmed  and  extended  by  several  mathematical  astronomers, 
notably  by  Louis  Fabry.  Fabry  concluded:  If  the  Sun 
travels  through  the  stellar  system  and  the  comets  come  to  the 
Sun  from  interstellar  space,  then  the  comets  should  all  move 
in  hyperbolas — differing  from  the  parabola  the  more  as  the 
velocity  of  the  Sun  through  space  is  the  greater. 

What  are  the  facts  of  observation?  Of  347  comet  orbits 
fairly  well  determined 

(a)  60  are  certainly  elliptic; 

(b)  275  are  approximately  parabolic  ; 

(c)  12  or  fewer  are  slightly  hyperbolic; 

(d)  None  are  strongly  hyperbolic. 

Now  it  has  been  shown  by  Thraen,  Fayet  and  Fabry  in  the 
last  two  decades  that  several  of  the  twelve  orbits  thought  to  be 
hyperbolic  were  not  really  so,  but  that  they  owed  their  reputa- 
tions to  poor  or  insufficient  observations,  or  to  errors  in  the 
computations,  and  that  all  of  the  genuine  hyperbolas  save  one 
acquired  their  hyperbolicity  after  the  comets  concerned  came 
under  the  disturbing  influences  of  our  planets.  Five  years 
ago  (1911)  Stromgren  was  able  to  show  that  the  one  out- 
standing hyperbolic  orbit  was  caused,  in  the  same  way,  by  the 
disturbing  attractions  of  the  planets.  The  original,  undis- 
turbed orbit  of  every  one  of  the  so-called  hyperbolic  comets 
was,  therefore,  an  ellipse.  Fayet  has  further  shown  that  a 
very  great  majority  of  the  orbits  which  had  been  observed  to 
be  sensibly  parabolic  when  the  comets  were  near  the  planets 
and  Sun  were  clearly  elliptic  when  the  comets  were  still  far 
out  from  the  Sun ;  that  is,  as  these  comets,  moving  in  elliptic 
orbits,  came  in  toward  the  planets  and  Sun,  the  attractions  of 
the  planets  made  their  orbits  approach  closely  to  the  parabolic 
form.  There  is  no  reason  to  doubt  that  far  out  in  the  domain 
of  the  Sun  the  comets  all  approach  in  elliptic  orbits ;  but  that 
when  the  attractions  of  one  or  more  of  our  planets  upon  them 
become  appreciable,  some  of  the  orbits  are  changed  into  shorter 
ellipses,  others  are  changed  into  ellipses  so  long  that  it  is 


34  THE  ADOLFO  STAHL  LECTURES 

difficult  to  distinguish  them  from  parabolas,  and  many  orbits 
are  changed  to  the  hyperbolic  form.  Those  comets  whose 
orbits  are  thus  thrown  into  the  hyperbolic  form  will  leave  the 
solar  system  and  travel  out  through  the  stellar  system. 

3.  A  statistical  study  of  comet  orbits  made  by  Leuschner 
a  decade  ago  bears  upon  this  question.  He  found  that  prior 
to  1755  ninety-nine  per  cent  of  all  comets  were  said  to  move  in 
parabolic  orbits,  but  that  only  fifty-four  per  cent  of  comets 
between  1846  and  1895  were  said  to  move  in  orbits  approxi- 
mately parabolic ;  and,  secondly,  that  of  comets  under  observa- 
tion less  than  one  hundred  days,  sixty-eight  per  cent  were  said 
to  be  parabolas,  whereas  of  those  observed  from  eight  months 
to  seventeen  months,  only  thirteen  per  cent  have  orbits  approxi- 
mately parabolic.  These  facts  point  to  the  conclusion  that 
when  comets  are  observed  inaccurately,  as  of  old,  and  in  only 
a  short  section  of  their  orbits,  parabolic  orbits  satisfy  the 
observations  within  the  limits  of  the  errors  unavoidably 
attaching  to  those  observations ;  but  that  when  comets  are 
observed  accurately  and  for  a  long  stretch  of  time,  nearly  all 
are  found  to  be  moving  in  ellipses.  Most  of  the  ellipses  are 
of  course  extremely  long  ones. 

If  comets  starting  substantially  at  rest  came  from  a  very 
great  distance  away  from  our  Sun,  say  one-hundredth  the 
distance  of  the  nearest  star,  which  we  think  is  decidedly  within 
the  sphere  of  our  Sun's  attraction,  they  would  move  in  ellipses 
so  elongated  that  we  could  not  hope  to  distinguish  them  from 
parabolas.  Their  periods  of  revolution  would  be  nearly  one 
hundred  and  fifty  thousand  years.  Yet  they  would  be  members 
of  our  solar  system,  subject  to  the  Sun's  attraction,  and  unless 
disturbed  by  some  other  body  or  bodies,  they  would  return 
again  and  again  to  the  center  of  the  system. 

The  work  of  Carrington,  Schiaparelli,  Fabry,  Fayet, 
Stromgren  and  Leuschner  and  of  many  others  has  left  no 
room  for  doubt  that  comets  are  bona  fide  members  of  our 
solar  system.  The  materials  composing  the  great  majority  of 
comets  spend  most  of  their  time  in  regions  far  removed  from 
the  Sun  and  its  planets,  as  our  little  distances  in  the  planetary 
system  go,  but  close  to  the  Sun  in  terms  ot  the  magnificent 
distances  which  separate  our  Sun  from  the  other  suns.  They 


WHAT  WE  KNOW  ABOUT  COMETS 


35 


are  moving  in   closed  orbits   around  our   Sun  and   traveling 
through  space  along  with  our  Sun.4 

Besides  the  comets  which  go  out  on  extremely  elongated 
orbits  to  great  distances  from  the  Sun,  there  are  about  fifty 
elliptic  comets  which  are  closely  related  in  one  sense  to  some 
of  our  planets.  About  three  dozen  are  in  the  so-called  Jupiter- 


FIG.  2.    JUPITER'S  FAMILY  OF  COMETS  (UP  TO  1893). 

family  of  comets.  The  orbits  of  all  those  discovered  up  to 
1893  are  represented  in  Fig.  2.  It  is  seen  that  the  outer  parts 
of  all  of  them — the  aphelia — are  in  the  vicinity  of  Jupiter's 
orbit.  Similarly,  there  are  a  few  comets  related  to  Saturn's 
orbit,  a  few  to  the  orbit  of  Uranus,  and  six  comets  to  the  orbit 


4  Those  who  would  like  to  look  more  thoroughly  into  this  question  are  strongly 
advised  to  read  Schiaparelli's  paper  on  "Orbites  cometaires,  Courants  cosmiques, 
Meteorites,"  in  Bulletin  Astronomique,  27,  194-205  and  241-254,  1910.  It  em- 
bodies some  points  of  view  slightly  different  from  those  presented  by  me.  The 
technical  contributions  by  Fabry,  Fayet  and  Stromgren  are  extensive  and  of  a  high 
order  of  merit;  and  students  of  comets  cannot  afford  to  neglect  them. — W.  W.  C. 


36  THE  ADOLFO  STAHL  LECTURES 

of  Neptune,  one  of  the  latter  being  Halley's  comet.  The 
Jupiter  comets  have  periods  lying  between  three  and  nine 
years,  and  the  Neptune  comets  complete  their  circuits  in  from 
sixty  to  eighty-one  years. 

What  has  been  the  history  of  these  short-period  comets? 
H.  A.  Newton  and  other  investigators  have  shown  that  it 
would  be  impossible  for  great  numbers  of  comets,  such  as 
have  been  observed,  to  move  through  the  solar  system,  without 
a  certain  proportion  having  their  orbits  changed  into  short- 
period  elliptic  orbits.  It  is  the  accepted  view  that  the  short- 
period  comets  have  been  captured,  so  to  speak,  by  the  combined 
attractions  of  the  Sun  and  one  of  the  planets  in  each  case. 
The  chances  of  capture  by  the  planets  are  greatest  when  the 
approaching  bodies  are  moving  in  orbits  which  lie  in  planes 
most  nearly  coincident  with  the  plane  of  the  planetary  system, 
and  when  their  motions  around  the  Sun  are  from  west  to  east. 
Newton's  analysis  of  the  problem  led  to  the  conclusion  that 
five  or  six  times  as  many  captured  comets  should  move  in  the 
direct  sense,  west  to  east,  as  in  the  retrograde  sense,  east  to 
west.  Now  the  only  comets  with  periods  less  than  one  hundred 
years  which  are  revolving  around  the  Sun  in  the  retrograde 
direction  are  Halley's  comet,  period  seventy-six  years,  and 
Comet  18661,  period  thirty-three  years.  The  three  dozen  mem- 
bers of  the  Jupiter- family  revolve  from  west  to  east  without 
exception.  That  the  motion  in  the  short-period  orbits  is  so  uni- 
versally from  west  to  east  finds  the  most  probable  explanation 
in  the  view  that  the  cometary  materials,  when  they  were  far- 
thest from  the  Sun,  long  before  they  approached  the  region  of 
the  planets  and  the  Sun,  already  had  a  slow  motion  from  west 
to  east,  the  motion  of  the  parent  mass  of  matter  from  which 
the  solar  system  itself  was  developed.  The  French  astronomer, 
Faye,  on  the  assumption  that  comets  have  originated  in  the 
outer  parts  of  a  rotating  mass  which  has  developed  into  the 
solar  system,  came  to  the  conclusion  that  comets  should  move 
prevailingly  in  the  direct  sense  when  their  orbit  planes  do  not 
differ  greatly  from  the  orbit  planes  of  the  planets,  but  that 
those  whose  orbit  planes  make  great  angles  with  the  plane  of 
the  solar  system  should  show  no  preference  for  the  direct  over 
the  retrograde  motion.  These  theoretical  results  are  in  good 
accord 'with  the  observed  facts. 


FIG.  1 — Comets'  tails  lag  behind  the  line  joining  the  Sun  (S)  and  the 
comets'  nuclei.  Orbital  motion  is  carrying  the  nucleus  of  the  comet 
to  the  right. 


FIG.  2—  Diagram  illustrating  the  three  principal  types  of  tails  of  comets. 
Orbital  motion  is  carrying  the  nucleus  to  the  left.    The  Sun  is  below. 


PLATE  VIII. 


WHAT  WE  KNOW  ABOUT  COMETS  37 

Our  second  question  is,  What  are  comets  ? 
Comets  have  certain  characteristic  features : 

1.  There  is  always  a  head,  or  coma  as  it  is  sometimes 
called,  a  shining  mass  of  hazy,  nebulous  matter.    The  head  is 
sometimes   circular   in   outline,   more   frequently   elliptical   or 
nearly  so,  but  again  it  is  oval  on  the  edge  facing  the  Sun  and 
merges  insensibly  into  the  tail  on  the  side  opposite  the  Sun 
(Plates  VI  and  VII).     The  sizes  of  comet  heads  vary  enor- 
mously.    One  less  than  ten  thousand  miles  in  diameter  would 
be  most  unusual  and  generally  would  escape  discovery.     The 
head  of  the  great  comet  of  1811  was  at  one  time  more  than  a 
million  miles  in  diameter.     The  head  of  the  great  comet  of 
1882,  which  many  of  us  enjoyed  seeing,  was  for  a  long  time 
about  one  hundred  and  fifty  thousand  miles  in  diameter.     It 
is  a  curious  fact  that  the  heads  of  comets  in  general  contract 
in  size  as  they  approach  the  Sun  and  expand  as  they  recede 
from  the  Sun.     Encke's  periodic  comet,  which  has  been  ob- 
served on  many  returns,  frequently  had  a  diameter  of  two  hun- 
dred and  fifty  thousand  miles  or  more  when  the  comet  was  at 
a  great  distance  from  the  Sun,  whereas  the  diameter  of  the 
head  reduced  to  ten  thousand  or  fifteen  thousand  miles  when 
the  comet  was  nearest  the  Sun.    Before  the  disappearance  into 
distant  space  the  head  resumed  its  original  dimensions.     A 
satisfactory  explanation  of  the  contraction  and  expansion  of 
the  heads  of  comets  has  not  been  found. 

2.  Near  the  center  of  the  head  of  the  comet  there  is  usually 
a  brilliant,  star-like  point  which  we  call  the  nucleus   (Fig.  1, 
Plate  VII).     This  is  the  point  upon  which  accurate  measures 
are  made  when  it  is  a  question  of  determining  the  position  and 
the  orbit  of  the  comet.    In  general  the  nuclei  are  most  sharply 
defined  for  those  comets  which  have  come  in  from  great  dis- 
tances upon  orbits  nearly  parabolic,  and  the  nuclei  are  frequent- 
ly hazy,  poorly  defined,  and  sometimes  entirely  lacking,  in  the 
comets  composing  Jupiter's  comet  family.     Occasionally  there 
is  a  double,  a  triple,  or  a  quadruple  nucleus,  a  division  undoubt- 
edly connected  with  the  disintegration  or  breaking  up  of  the 
comet  into   smaller  masses.     The  size  of  the  nucleus  varies 
greatly,  apparently  from  a  few  miles  up  to  several  thousand 
miles  in  diameter. 


38  THE  ADOLFO  STAHL  LECTURES 

3.  Most   comets   have  tails.     They   frequently   develop  to 
enormous  dimensions.     When  a  comet  is  observed  at  a  great 
distance  from  the  Sun,  only  the  head  and  nucleus  are  usually 
visible.    The  tail  develops  with  close  approach  to  the  Sun.    The 
tail  of  the  comet  of  1882  was  at  one  time  more  than  one  hun- 
dred million  miles  in  length ;  that  of  1843  was  at  one  time  two 
hundred  million  miles.     As  comets  recede  from  the  Sun,  the 
tails  diminish  in  extent  and  usually  disappear  long  before  the 
head  and  nucleus  are  lost  to  sight.     Several  of  the  Jupiter 
comets  do  not  have  visible  tails   (Fig.  2,  Plate  VII).     They 
appear  not  to  possess  in  abundance  the  materials  which  go  to 
form  comets'  tails. 

4.  When  comets  approach  relatively  close  to  the  Sun  the 
heads   frequently  throw  off  a  series  of  concentric   shells  or 
envelopes.     The  materials  composing  these  envelopes  appear 
to  be  expelled  from  the  head  and  toward  the  Sun  at  high  speed, 
but  these  speeds  of  approach  to  the  Sun  seem  to  be  gradually 
overcome  and  the  materials  turned  away  from  the  Sun  to  assist 
in  forming  the  tails  (Fig.  1,  Plate  VII). 

The  tails  of  comets,  it  is  well  known,  point  away  from  the 
Sun.  However,  the  popular  view  that  they  point  exactly  away 
from  the  Sun  is  seriously  in  error.  In  general  they  lag  behind 
the  line  passing  through  the  Sun  and  the  comet's  head  (Fig. 
1,  Plate  VIII).  There  can  be  no  doubt  that  they  point  away 
from  the  Sun  because  of  some  repulsive  force,  originating  in 
the  Sun,  which  acts  upon  the  minute  dust  particles  or  gas  mole- 
cules released  from  the  comet's  head.  It  takes  time  for  these 
particles  to  travel  out  millions  of  miles  from  the  head,  and, 
while  they  are  moving  out,  the  head  is  moving  forward  in  its 
orbit.  The  nucleus  obeys  the  gravitational  attraction  of  the 
Sun  absolutely,  so  far  as  observation  has  gone,  and  we  have 
no  reason  to  suspect  that  it  is  subject  to  an  appreciable 
repulsive  force.  The  particles  composing  the  outer  regions 
of  the  head  and  the  particles  composing  the  tail  are  doubtless 
attracted  by  the  gravitation  of  the  Sun  and  are  at  the  same 
time  driven  away  by  the  repulsion  of  the  Sun.  What  the 
particles  will  do  under  the  action  of  the  two  opposing  forces 
depends  upon  the  ratio  of  these  forces.  If  the  repulsive  force 
is  vastly  stronger  than  the  attracting  force  the  particles  will 


WHAT  WE  KNOW  ABOUT  COMETS  39 

travel  out  from  the  head  with  great  and  increasing  speed  and 
form  a  tail  pointing  nearly  away  from  the  Sun ;  that  is,  it  will 
lag  behind  very  little.  If  the  attracting  and  repelling  forces 
acting  upon  another  group  of  particles  are  not  very  unequal 
those  particles  will  form  a  second  tail  having  considerable  lag. 
If  the  repulsive  force  is  very  weak  with  reference  to  the  Sun's 
attractive  force  upon  a  third  group  of  particles,  they  will  form 
a  short  tail  that  lags  very  far  behind.  The  forms  and  positions 
of  comet  tails  were  studied  extensively  by  Bredichin,  who 
found  that  there  were  three  classes  of  tails,  corresponding  to 
three  fairly  definite  ratios  of  repulsive  to  attractive  forces,  as 
indicated  by  three  different  degrees  of  lagging  behind  the  line 
joining  the  Sun  and  the  head  (Fig.  2,  Plate  VIII). 

Bredichin  determined  that  the  long  slender  tails,  observed 
in  a  few  comets,  which  lag  behind  only  slightly  are  the  result 
of  a  repulsive  force  twelve  to  fifteen  times  as  intense  as  the 
attractive  force.  He  found  another  class  of  comet  tails,  of 
medium  lag,  for  which  the  repulsive  forces  were  from  2.2  to 
0.5  times  the  attractive  forces.  Another  class  of  tails,  short 
and  bushy,  with  very  strong  lag,  were  explainable  on  the 
assumption  that  the  repulsive  forces  were  relatively  weak,  from 
0.3  to  0.1  of  the  attractive  forces. 

In  some  comets  only  one  of  these  three  classes  of  tails  is 
present,  and  again  in  one  and  the  same  comet  all  of  the  classes 
may  be  present  at  the  same  time. 

That  there  is  outward  motion  of  the  tail  materials  admits  of 
no  doubt.  It  is  not  uncommon  for  the  tail  materials  of  one 
night  to  be  driven  off  into  space,  scattered  and  lost  to  sight,  and 
for  an  entirely  new  tail  to  take  its  place  by  the  following  night. 
A  comet's  tail  is  constantly  forming  and  moving  out.  The  tails 
of  Comet  Rordame  (Plate  IX)  photographed  by  Hussey  on 
two  successive  nights,  July  12  and  13,  1893,  have  no  points  of 
resemblance.  The  streamers  composing  the  tail  on  one  night 
are  fairly  straight,  regular,  and  rather  faint.  The  tail  of  the 
following  night  is  very  much  broken,  there  are  several  fairly 
well-defined  nuclei,  and  it  is  brighter  than  the  tail  of  the 
12th.  Two  photographs  of  this  comet  were  fortunately 
made  on  the  second  night,  with  a  time  interval  of  three- 
quarters  of  an  hour.  A  comparison  of  the  positions  of  the 


40  THE  ADOLFO  STAHL  LECTURES 

three  nuclei  on  the  two  plates  showed  that  they  had  moved 
outward  from  the  head  with  great  speed  during  the  interval. 
The  nucleus  nearest  the  head  had  traveled  out  with  a  speed  of 
forty-four  miles  per  second,  the  next  nucleus  with  a  speed  of 
fifty-two  miles  per  second,  and  the  one  still  farther  out  with  a 
speed  of  fifty-nine  miles  per  second.  Here  are  two  photo- 
graphs of  Comet  Brooks  (Plate  X)  made  on  October  21  and 
October  22,  1893,  by  Barnard.  The  structure  of  the  tail  on 
the  first  photograph  is  not  at  all  the  structure  on  the  second. 
The  tail  of  the  first  night  has  been  scattered  to  invisibility  and 
an  absolutely  new  tail  has  replaced  it.  The  outward  motion 
of  well-defined  tail  structure  has  been  measured  for  many 
comets.  Here  is  a  series  of  measures  made  by  Curtis  upon 
points  in  the  tails  of  Halley's  comet. 

AVERAGE  VELOCITIES  OF  RECESSION,  FROM  THE  HEAD,  OF  MATTER  IN  THK 

TAIL  OF  HALLEY'S  COMET 
Date,  1910  Mean  Distance  from  Head     Average  Velocity 

May  23 800  miles          0.6  miles  per  sec. 

May  27-28  400,000  miles          8     miles  per  sec. 

May  25-26  930,000  miles        12     miles  per  sec. 

June   2-3    1,360,000  miles        20     miles  per  sec. 

May    28-29    1,730,000  miles        23      miles  per  sec. 

June   6   2,200,000  miles        27      miles  per  sec. 

May  26-27  2,500,000  miles        24      miles  per  sec. 

May  30-31  6,600,000  miles        45     miles  per  sec. 

June   7-8   8,400,000  miles        57     miles  per  sec. 

The  points  to  be  measured  wrere  not  well  defined,  and  the 
measures  could  not  be  accurate,  but  it  is  clear  that  high  speeds 
and  accelerated  speeds  prevailed.  The  tail  materials  start  out 
slowly  from  the  head,  and  increase  their  speeds  with  the 
distance  from  the  head,  as  we  should  expect  of  motion  result- 
ing from  the  action  of  a  continuous  force  which  meets  with  no 
sensible  resistance. 

In  Plate  XI  are  reproductions  of  photographs  of  Halley's 
comet  made  by  Curtis  on  June  6  and  June  7,  1910.  A  semi- 
detached part  of  the  tail,  seen  on  the  photograph  of  June  6 
about  an  inch  above  the  head,  is  visible  about  two  and  a  half 
inches  above  the  head  on  the  photograph  of  June  7.  This 
structure  was  first  observed  by  Curtis  shortly  after  it  had 
emerged  from  the  central  part  of  the  head  on  June  4,  and  it 


PLATE  IX.    COMET  RORDAME  ON  JULY  12  AND  JULY  13,  1893. 

Photographs  by  W .  J.  Hussey. 
The  camera   followed  the  nucleus  of  the  comet,  and  the   stars   "trailed." 


WHAT  WE  KNOW  ABOUT  COMETS 


41 


f — Haaius  Vector 


.6.740    Mt.  Baoilton 


.6.475    Cordate 


.7.836    Chrigtchurch, 


was  recorded  on  the  photographs  secured  by  a  great  many 
observatories  in  the  following  four  days,  as  the  rotation  of 
the  Earth  brought  the  comet  successively  into  position  for 
observation  at  the  different  observatories.  The  times  when  the 
lower  point  of  the  structure  had  certain  positions  are  indicated 
in  Fig.  3.  The  tail  did  not  seem 
to  lag  behind  the  position  of  the 
radius  vector — the  line  passing 
through  the  Sun  and  the  comet's 
nucleus — because  the  observers  on 
those  days  were  nearly  in  the 
plane  of  the  comet's  orbit  and  the 
lag  of  the  tail  was  toward  the  ob- 
servers. The  velocity  with  which 
the  structure  moved  out  in  the  tail 
was  strongly  accelerated  with  the 
passing  of  time,  as  may  be  seen 
from  the  chart.  The  constant  loss 
of  materials  dispelled  along  the 
tail  would  seem  to  require  that 
comets  in  general  grow  fainter 
with  time.  This  is  the  logical  con- 
clusion, and  the  observational  evi- 
dence for  it  is  undoubted  in  many 
of  those  comets  which  return 
again  and  again  to  the  region  of 
the  Sun.  Nearly  all  of  the  Jupiter 
comets  have  a  hazy,  washed-out 
appearance.  Several  of  them  do 
not  develop  tails,  as  if  their  supply 
of  tail  materials  had  already  been 
exhausted  by  expulsion  as  former 
tails.  Others  of  them  develop  only 
very  short  tails,  and  several  short- 
period  comets  have  entirely  disap- 
peared. To  this  phase  of  the  sub- 
ject we  shall  return. 


-7.735    Mt.  Hamilton 


.7.505    CoVdoba 


!86  flaimt,    Syria 

.7.057  Dairea 

.6.997  Tokyo 

.6.632  Honolulu 

.6.737  ut.  Hamilton 

-6.  '665  Yerlcas 

.6.494  Cordoba 


-6.064    Dairan 


.783    Ut.  Hamilton 


FIG.  3.     SUCCESSIVE  POSITIONS 
OF  THE  INNER  END  OF  A  DE- 
As    to    the    nature   of    the    re-   TACHED     TAIL     OF     HALLEY'S 
pulsive  force  responsible  for  com-  COMET,  JUNE  4-8,  1910. 


42  THE  ADOLFO  STAHL  LECTURES 

ets'  tails :  It  was  long  thought  to  be  electrical,  arising  from 
a  strong  electrical  field  about  the  Sun  and  from  electric 
charges  of  the  same  sign  on  the  particles  composing  the  tail. 
The  idea  is  in  part  purely  speculative,  but  the  giving  of 
serious  consideration  to  it  is  justified  because  of  the  fact  that 
much  of  the  light  of  comets  seems  to  arise  from  electrical 
conditions  in  them.  The  idea  may  be  wrong  in  toto,  or  an 
electric  repulsive  force  may  be  one  of  two  or  more  forces 
which  are  acting.  It  can  hardly  be  the  only  force  involved. 

Clerk-Maxwell  half  a  century  ago,  from  pure  theory,  and 
Lebedew  and  Nichols  and  Hull  some  fifteen  years  ago,  from 
experimental  evidence  admitting  of  no  doubt,  showed  that 
when  light  energy  falls  upon  a  surface  it  presses  against  that 
surface ;  very  feebly  it  is  true,  but  it  will  cause  the  body  pressed 
upon  to  move  if  that  body  is  not  too  massive.  In  this  respect 
light-pressure  repulsion  and  electric  repulsion  should  act  much 
alike.  These  repulsions  are  effective  in  proportion  to  the 
surface  areas  of  the  bodies  acted  upon,  whereas  gravitation 
pulls  those  bodies  with  a  force  proportional  to  their  masses. 
Now  the  surface  of  a  body  is  proportional  to  the  square  of  its 
dimensions,  whereas  gravity  acts  in  proportion  to  the  cube  of 
its  dimensions.  The  smaller  a  body  is,  the  more  surface  it  has 
in  proportion  to  its  mass.  Electric  and  radiation-pressure 
repulsions  will  therefore  act  more  efficiently  upon  very  small 
particles  than  upon  large  ones.  A  cubse  of  water  one  centi- 
meter on  each  edge  would  be  drawn  by  the  Sun's  gravitational 
action  ten  thousand  times  as  strongly  as  the  pressure  of  the 
Sun's  rays  falling  upon  that  body  would  repel  it.  But  a  cube  of 
water  only  0.001  of  a  mm.  on  each  edge  would  be  in  equilib- 
rium under  the  Sun's  gravitational  attraction  and  the  Sun's 
light-pressure  repulsion.  A  cube  of  water  less  than  0.001 
mm.  would  actually  be  driven  rapidly  away  from  the  Sun. 
The  equilibrium  diameter  for  little  spheres  of  water,  according 
to  Nichols  and  Hull,  is  0.0015  mm.  Now  as  light  energy  is 
traveling  along  with  a  speed  of  186,000  miles  a  second,  we 
should  expect  particles  of  matter  considerably  smaller  than  the 
equilibrium  size  to  travel  away  from  the  Sun  with  great  and 
rapidly  increasing  speeds.  These  speeds  would  be  the  greater 
for  particles  smaller  and  smaller  until  a  certain  limit  of  size 


PLATE  X.     COMET  BROOKS  ON  OCT.  21  AND  OCT.  22,   1895. 
Photographs  by  E.  E.  Barnard. 


WHAT  WE  KNOW  ABOUT  COMETS  43 

with  reference  to  the  wave-length  of  light  is  reached,  after 
which  the  light  would  be  diffracted  without  transmitting  so 
large  a  proportion  of  its"  repulsive  energy  to  the  particles. 
These  limits  of  efficiency  were  determined  by  the  lamented 
Schwarzschild. 

The  resistance  of  cometary  particles  is  evidently  also  a 
function  of  the  specific  gravity  of  the  particles.  The  figures 
which  we  have  quoted  are  for  water,  density  1.  We  can 
scarcely  doubt  that  radiation  pressure  is  an  important  force, 
perhaps  the  chief  force,  perhaps  the  only  force  responsible 
for  the  driving  out  of  the  materials  of  comets'  tails.  Parti- 
cles of  solid  matter  or  gas  molecules  of  three  different  classes 
of  sizes  might  be  responsible  for  the  three  main  classes  of  com- 
ets' tails.  More  probably  materials  of  three  different  classes 
of  density  compose  the  three  classes  of  tails.  Bredichin  called 
these  three  classes  the  hydrogen,  the  hydrocarbon  and  the 
iron  tails.  The  atomic  weights  of  these  three  substances  give 
to  their  atoms  or  molecules  about  the  right  mobility,  under 
equal  pressure  upon  all,  to  explain  the  lags  of  the  three  classes 
of  tails.  Unfortunately  it  is  far  from  certain  that  hydrogen 
exists  in  comets,  and  iron  has  been  reported  for  only  one  comet. 

The  hoods  or  envelopes  (Fig.  1,  Plate  VII)  which  form  the 
outer  strata  of  the  heads  of  comets  which  come  close  to  the  Sun 
are  very  interesting.  It  is  the  prevailing  view  that,  when  a  comet 
approaches  the  Sun,  the  solar  heat  falling  upon  that  surface  of 
the  comet  which  faces  the  Sun  generates  or  liberates  the  gases 
and  vapors  which  have  been  contained  in  or  between  the  more 
solid  parts  of  the  comet;  and  being  liberated,  in  effect,  under 
pressure,  the  materials  at  first  travel  toward  the  Sun  with 
considerable  speed.  The  Sun's  repulsive  force  acts  upon  these 
jets  and,  overcoming  the  forward  motion  of  the  materials, 
it  eventually  turns  them  back  along  the  tail.  Those  phenomena 
have  been  observed  many  times. 

There  is  a  great  variety  of  comet  spectra,  indicating  as 
great  a  variety  of  cometary  contents  or  conditions.  In  some 
cases  the  spectrum  seems  almost  wholly  continuous,  as  in 
Holmes's  comet  of  1892;  in  others  the  light,  when  passed 
through  the  spectroscope,  falls  almost  wholly  into  isolated 
bright  lines  or  bands,  as  in  Morehouse's  comet  of  1908.  Other 


44  THE  ADOLFO  STAHL  LECTURES 

spectra  are  a  combination  of  continuous  and  bright-line  light 
(Fig.  1,  Plate  XII).  The  spectrum  of  the  nucleus  seems  to  be 
always  continuous,  or  continuous  except  for  absorption  lines. 
In  some  of  the  brighter  comets  the  nucleus  spectrum  as  photo- 
graphed contains  the  well-known  absorption  lines  visible  in 
the  Sun's  spectrum.  These  observations  indicate  that  the 
nucleus  is  shining,  at  least  mainly,  by  reflected  sunlight.  In 
most  comets  the  continuous  spectrum  is  too  faint  to  let  us 
photograph  it  and  thus  to  prove  the  presence  or  absence  of  the 
solar  absorption  lines.  The  continuous  spectrum  in  many 
comets  extends  also  to  the  head,  or  at  least  to  the  inner  strata 
of  the  head.  This  may  or  may  not  mean  reflected  sunlight.  It 
may  mean  some  other  form  of  luminescence  which  yields  a 
continuous  spectrum.  The  greater  parts  of  the  heads  of  comets 
and  those  parts  of  the  tails  of  comets  which  are  close  to  the 
heads  nearly  always,  and  perhaps  in  every  case,  give  a 
characteristic  spectrum  of  bright  bands,  which  were  for  several 
decades  called  the  hydrocarbon  bands.  Observations  of  recent 
years  have  made  it  probable  that  this  spectrum  does  not 
indicate  a  combination  of  hydrogen  and  carbon,  but  that  it 
is  either  one  of  the  low-pressure  carbon  vapor  bands  or  that  it 
results  from  one  of  the  compounds  of  carbon  and  oxygen, 
preferably  from  carbon  monoxide.  The  lines  and  bands  of 
cyanogen — a  nitrogen  compound — and  of  carbon  are  present 
without  any  question  in  the  heads  and  inner  tails  of  many  com- 
ets. Several  observers  have  reported  that  the  so-called  hydro- 
carbon spectrum  of  the  heads  and  inner  tails  extends  far  out 
into  the  tails.  This  may  have  been  true  for  the  cases  reported, 
but  recent  observations  are  casting  doubt  upon  the  presence  of 
that  spectrum  in  the  outer  extensions  of  comet  tails.  Improved 
methods  of  photographing  comet  spectra  were  applied  to  the 
bright  comets,  Daniels  of  1907  and  Morehouse  of  1908, 
especially  by  Deslandres,  Evershed  and  Chretien,  with  the 
result  that  their  tail  spectra  were  proved  to  be  very  different 
from  the  prevailing  spectra  of  comets'  heads  and  inner  tails. 
Fowler  has  succeeded  in  duplicating  the  tail  spectra  of  these 
two  comets,  in  his  laboratory,  with  remarkable  agreement  (  Fig. 
2,  Plate  XII),  by  photographing  a  cathode  spectrum  of  carbon 
monoxide  in  a  tube  reduced  to  pressure  not  exceeding  0.01 


WHAT  WE  KNOW  ABOUT  COMETS  45 

mm.  At  higher  pressure  than  this  he  obtained  the  so-called 
hydrocarbon  spectrum,  but  it  was  not  certain,  and  in  fact  it 
was  improbable,  that  there  was  any  hydrogen  in  the  tube.  The 
presence  of  carbon  and  nitrogen  in  comets  is  certain,  the 
presence  of  oxygen  is  probable,  and  the  presence  of  hydrogen 
is  doubtful. 

The  comets  which  have  approached  very  close  to  the  Sun 
turned  to  a  yellowish  orange  in  color  and  remained  so  while 
in  the  vicinity  of  the  Sun,  because  the  yellow  light  of  sodium 
then  developed  strongly  in  them,  apparently  by  virtue  of  the 
intense  heating  of  the  cometary  matter  by  the  Sun's  rays.  This 
happened  with  the  Wells  comet  of  1882,  the  great  comet  of 
September  and  October,  1882,  the  brilliant  comet  in  January, 
1910,  and  others.  When  the  September,  1882,  comet  was  only 
a  few  hundred  thousand  miles  from  the  Sun,  Copeland  and 
Lohse  observed  not  only  the  sodium  lines  but  half  a  dozen  other 
bright  lines  which  they  concluded  were  well-known  iron  lines. 

What  is  the  origin  of  the  light  which  gives  bright  lines  and 
bands?  The  sodium  lines  certainly,  and  the  iron  lines  if 
actually  observed,  were  no  doubt  due  to  the  vapors  of  those 
elements  having  been  rendered  incandescent  under  the  intense 
heat  or  other  influence  of  the  Sun.  Strangely  enough,  when 
the  brilliant  sodium  comets  approached  the  Sun,  the  carbon 
bands,  which  had  previously  been  prominent,  disappeared  and 
remained  invisible  until  the  comets  had  receded  to  a  consider- 
able distance  from  the  Sun  and  the  sodium  lines  were  no 
longer  in  evidence.  These  observations,  it  should  be  said,  were 
made  by  visual  means.  The  photographic  observations  of 
recent  years  have  been  much  more  efficient  in  detecting  the 
sodium  lines  and  carbon  bands  when  these  are  faint.  The  car- 
bon light  could  scarcely  be  generated  by  heat  action,  for  if  so 
the  carbon  bands  should  have  been  in  evidence  during  the 
time  that  the  comet  was  passing  nearest  to  the  Sun.  Much 
more  probably  the  bright-line  spectra  of  the  head  and  tail  are 
of  electrical  origin,  or  fluorescent.  This  phase  of  the  subject 
is  technical,  and  to  some  extent  speculative,  and  we  can  not 
profitably  pursue  it  further  on  this  occasion. 

A  certain  proportion  of  the  light  of  many  comets  is 
slightly  polarized.  The  interpretation  of  this  phenomenon  is 


46  THE  ADOLFO  STAHL  LECTURES 

that  a  fraction  of  the  light  of  the  heads  and  of  the  inner  tails 
of  comets  is  sunlight  diffracted  by  minute  dust  particles  or  gas 
molecules  in  the  comet  structure. 

Returning  to  the  subject  of  the  disintegration  and  dis- 
appearance of  comets : 

A  small  comet  was  discovered  by  Montaigne  in  1772.  A 
comet  was  discovered  by  Pons  in  1805.  A  comet  was  dis- 
covered by  Biela  in  1826.  Biela  computed  the  orbit  of  his 
comet  and  found  it  to  be  moving  in  an  ellipse  of  period  six 
and  a  half  years,  and  he  proved  that  the  three  comets  dis- 
covered respectively  by  Montaigne,  Pons  and  himself  were 
identically  the  same  comet.  Biela's  comet  was  rediscovered  in 
1832,  almost  precisely  in  its  expected  place.  The  next  return 
was  missed  because  the  body  was  not  in  good  position  for 
observing.  It  was  rediscovered  in  1845,  when  it  was  seen  to 
consist  of  two  comets  moving  side  by  side  on  orbits  almost 
identical.  In  1852  both  comets  were  reobserved,  but  farther 
separated  than  they  had  been  in  1845.  The  comet  was  searched 
for  at  the  proper  times  for  several  later  returns,  but  it  was 
never  seen  again.5 

Kirkwood  published  in  1872  a  list  of  eight  comets  which 
had  divided  in  a  similar  manner  and  disappeared. 

A  number  of  other  comets  have  completely  disappeared, 
though  their  orbits  were  very  well  determined.* 

This  brings  us  to  another  interesting  phase  of  our  subject. 

The  Perscid  meteors  were  with  us  again  last  August. 
Many  of  them  have  been  seen  every  year  for  several  decades. 
They  are  usually  most  numerous  on  the  nights  of  August  9, 
10  and  11.  Predictions  concerning  meteors  are  somewhat 
risky,  but  so  faithfully  have  the  Perseids  come  every  August 
that  I  have  no  doubt  an  observer  on  those  nights  of  August, 
next  year,  from  midnight  on  to  daylight,  will  see  dozens  of 
meteors  whose  paths  traced  backwards  would  pass  through 
a  small  area  in  the  constellation  of  Perseus.  In  1866  Schia- 
parelli  computed  the  orbit  of  the  Perseid  meteors  and  noticed 
that  it  was  essentially  identical  with  the  orbit  of  Comet  1862III. 
Here  are  the  elements  of  the  two  orbits : 

5  One  of  the  components  of  the  Biela  comet  may  have  been  observed  for  a 
few  hours  from  Madras  in  1872. 


PLATE   XI.     HALLEY'S   COMET,  JUNE  6  AND  JUNE  7,   1910. 
Photographs  by  H.  D.  Curtis. 


WHAT  WE  KNOW  ABOUT  COMETS  47 

Meteors  of  August 

Orbits  of  9,  10,  11  Comet  1862III 

Perihelion  passage  July  23.62  August  22.9 

Longitude  of  perihelion 343°  38'  344°  41' 

Ascending  node  138     16  137    27 

Inclination    63       3  66      25 

Perihelion  distance  0.9643  0.9626 

Period  of  revolution 105  years?  123.4 

Direction   of   motion  retrograde  retrograde 

The  difference  in  the  two  perihelion  times  does  not  mean 
that  their  orbits  were  different  even  to  the  minutest  degree, 
but  only  that,  moving"  on  the  same  orbit,  they  reached  the 
point  nearest  the  Sun  at  slightly  different  times;  that  is,  the 
meteors  traveled  over  the  orbit  a  little  in  advance  of  the 
comet.  The  revolution  period  assigned  to  the  meteors  is 
subject  to  considerable  error  because  it  is  not  possible  to 
observe  the  paths  of  the  meteors  with  great  accuracy. 

There  were  rich  and  startling  showers  of  meteors  on 
November  12,  1799,  and  on  November  12-13,  1833.  H.  A. 
Newton  examined  the  literature  of  meteoric  falls  and  found 
that  many  similar  showers  had  been  observed  at  intervals 
of  thirty-three  years  running  back  several  centuries  to  902 
A.D.,  "the  year  of  the  stars,"  and  he  confidently  predicted  that 
another  great  shower  would  occur  on  November  13-14,  1866. 
His  prediction  was  abundantly  verified.  Early  in  1867  Schia- 
parelli  and  Le  Verrier  independently  computed  the  orbit  of 
these  meteors,  and  Schiaparelli  and  Oppolzer  independently 
found  it  identical  with  the  orbit  of  Comet  18661.  Here  are 
the  elements  of  the  two  orbits : 

Meteors  of  Novem- 

Orbits  of                                           ber  13  Comet  18661 

Perihelion  passage  November  10.092  January  11.160 

Longitude  of  perihelion  56°  25  .9'  60°  28  .0' 

Ascending  node  231    28.2  231    26.1 

Inclination  17   44.5  17    18.1 

Perihelion  distance  0.9873  0.9765 

Eccentricity   0.9046  0.9054 

Semi-major  axis   10.340  10.324 

Period  of  revolution  33.250  years  33.176  years 

Direction  of  motion  retrograde  retrograde 

It  is  impossible  to  doubt  that  these  November  meteors  and  the 
comet  referred  to  were  traveling  in  the  same  orbit. 


48 


THE  ADOLFO  STAHL  LECTURES 


The  so-called  Lyra  meteors  are  visible  about  April  20  each 
year.  It  was  noticed  in  1867  by  Weiss  that  the  orbit  of  the 
Lyra  meteors  is  essentially  identical  with  that  of  Comet  18611. 

Biela's  comet,  to  which  we  have  referred,  when  last  seen  in 
1852,  as  a  double  comet,  was  expected  to  return  in  1866  and 
again  in  1872,  but  it  was  not  seen  then,  nor  later.  A  meteor 
shower  of  moderate  intensity  was  observed  on  November  27, 
1872,  moving  in  the  orbit  of  the  lost  comet. 


FIG.    4.     ORBITS    OF    METEORIC    SWARMS,    WHICH    ARE    KNOWN    TO    BE 
ASSOCIATED  WITH  COMETS. 

Not  to  dwell  upon  the  remarkable  identities  of  the  orbits 
of  the  four  meteor  swarms,  respectively,  with  the  orbits  of  the 
four  comets  (Fig.  4),  two  of  which  have  disappeared,  and 
the  other  two,  of  relatively  long  periods,  which  may  never 
return,  we  express  the  prevailing  opinion  of  astronomers  in 
saying  that  the  meteor  streams  have  actually  resulted  from 
the  disintegration  of  the  four  comets.  Alexander  Herschel 
has  prepared  a  list  of  seventy-six  meteor  streams  whose  orbits 
agree  fairly  closely  with  the  seventy-six  comet  orbits.  A  cer- 
tain proportion  of  the  suspected  identities  probably  represent 


FIG.  1 — Spectrum  of  Comet  Daniels,  1907. 


FiG.  2 — (a)  Ordinary  photograph  of  Comet  Morehouse.  (b)  Spectrum 
photograph  of  Comet  Morehouse  made  at  same  time  as  (a),  (c) 
Fowler's  spectrum  of  carbon  monoxide,  whose  principal  bands 
match  the  principal  spectrum  images  of  the  comet's  tail. 


PLATE  XII. 


WHAT  WE  KNOW  ABOUT  COMETS  49 

facts.  It  is  interesting  to  note  that  even  as  early  as  1861  the 
truth  of  the  situation  was  expressed  and  printed  by  Kirkwood : 

May  not  our  periodic  meteors  be  the  debris  of  ancient  but  now 
disintegrated  comets  whose  material  has  become  distributed  around 
their  orbits? 

It  was  in  this  connection  and  at  that  time  that  Kirkwood  was 
able  to  make  a  list  of  eight  comets,  each  of  which  had  divided 
into  two  or  more  parts  and  had  wholly  disappeared  from  the 
sight  of  observers. 

The  cause  of  the  disintegration  of  comets  is  not  far  to  seek. 
A  comet's  nucleus  is  thought  to  be  a  collection  or  cluster  of 
small  bodies,  such  as  have  been  observed  to  collide  with  our 
atmosphere  and  to  produce  the  meteor  showers.  They  are  held 
together,  so  to  speak,  while  they  are  far  away  from  the  Sun, 
because  of  their  own  very  small  but  sufficient  attraction  for 
each  other ;  but  when  they  come  within  our  planetary  system, 
and  especially  when  they  come  relatively  close  to  the  great 
planets  Jupiter  and  Saturn,  the  Sun  and  the  planets  attract  the 
nearer  particles  of  the  comets  more  strongly  than  they  do  the 
farther  particles.  The  nearer  particles  forge  ahead  on  smaller 
orbits,  the  farther  particles  lag  behind  on  larger  orbits,  and  in 
the  course  of  centuries  the  cometary  material  is  strewn  along 
a  great  stretch  of  the  orbit.  Other  separative  forces — of 
magnetic  or  electric  natures,  for  example — may  develop 
amongst  the  particles  composing  the  nucleus  as  a  comet 
approaches  the  Sun.  The  intensity  of  the  reflected  light  in  all 
parts  of  the  scattered  comet  structure  becomes  too  small  to  let 
us  see  the  remains  of  the  comet,  except  as  the  remnants  collide 
with  the  Earth's  atmosphere.  There  is  certainly  no  reason  to 
doubt  that  a  very  great  many  of  our  shooting  stars  are  the 
remains  of  disintegrated  comets.  Tens  of  millions  of  little 
meteors  enter  our  atmosphere  every  twenty-four  hours  and 
with  rare  exceptions  are  consumed  by  the  heat  of  friction  with 
the  atmosphere  when  they  rush  through  it  at  tremendous 
speeds.  The  gases  from  the  combustion  enter  the  atmosphere, 
and  the  ash  and  other  unconsumed  parts  fall  down  to  the 
Earth's  surface  in  due  time.  Accumulated  meteoric  dust  is 
found  in  the  perpetual  snows  at  the  tops  of  high  mountains, 
and  Sir  John  Murray  found  it  in  the  ooze  brought  up  from 


50  THE  ADOLFO  STAHL  LECTURES 

the  depths  of  the  oceans.  Whether  the  meteorites  which 
penetrate  our  atmosphere  and  are  found  and  placed  in  our 
museums  are  parts  of  ancient  comets  can  not  safely  be  asserted, 
but  it  seems  entirely  possible  that  some  of  them  are.  However, 
it  is  not  certain  that  any  meteorite  found  on  the  Earth  has 
come  from  a  meteor  stream  of  recognized  cometary  origin.  It 
is  pretty  well  established  that  many  of  the  sporadic  meteors 
which  plunged  into  our  atmosphere  were  traveling  on  hyper- 
bolic orbits. 

We  discover  only  a  certain  proportion  of  the  comets  which 
come  close  to  the  Sun  and  to  the  Earth.  The  numbers  which 
course  through  the  planetary  system  and  remain  undiscovered 
by  the  observers  on  the  Earth  must  be  exceedingly  great.  The 
supply  of  cometary  material  in  the  remote  outskirts  of  the 
planetary  system  must  be  enormous.  This  material  is 
probably  in  the  nature  of  remnants  of  the  nebula  or  other  mass 
of  matter  from  which  the  Sun,  its  planets  and  their  moons 
developed.  This  idea  is  to  a  certain  extent  speculative ;  but 
that  the  cometary  material  is  now  out  there  in  abundance  we 
can  not  doubt.  Much  of  it  naturally  consists  of  matter  in  the 
solid  state;  and,  the  Sun's  attraction  at  that  great  distance 
being  almost  zero,  neighboring  masses  could  slowly  come 
together  as  a  collection  of  small  solid  masses,  such  as  seem  to 
compose  the  nucleus  of  a  comet.  Such  a  nucleus  could  attract 
and  attach  to  itself  any  dust  particles  and  molecules  coming 
within  its  sphere  of  attraction.  These  might  well,  and  probably 
would,  include  a  collection  of  finely  divided  matter  that  had 
already  been  driven  off  in  the  tails  of  comets  which  in  earlier 
ages  had  visited  the  Sun.  The  materials  thus  collected  would  be 
attracted  by  the  Sun,  a  few  of  the  collections  would  eventually 
pass  comparatively  close  to  the  Sun,  a  few  of  the  latter  would 
be  discovered  as  comets,  and  a  part  of  the  finely  divided 
material  contained  in  them  would  be  driven  off  again  as 
comets'  tails  into  space,  possibly  to  return  many  times  in  the 
bodies  of  comets  coming  later  into  the  Sun's  neighborhood. 
Certain  of  these  bodies  would  come  so  close  to  the  planets  as 
to  have  their  orbits  transformed  from  very  long  ellipses  into 
very  short  ellipses.  Those  comets  would  be  disintegrated  and 
their  materials  be  widely  scattered.  We  have  seen  that  the 


WHAT  WE  KNOW  ABOUT  COMETS  51 

Earth  has  collided  with  such  materials,  and  the  Earth  is 
growing  slowly,  very  slowly,  through  the  deposition  of  the 
remains  upon  its  surface.  Probably  a  little  of  the  same 
materials  goes  likewise  to  other  planets  of  the  solar  system  and 
adds  slowly  to  their'  masses.  However,  an  insignificant  pro- 
portion of  the  materials  scattered  in  this  manner  through  the 
solar  system  is  thus  accounted  for,  and  the  remainder  doubtless 
revolves  around  the  Sun  in  ellipses,  probably  contributing  its 
share  of  reflected  sunlight  to  the  faint  glow  near  the  Sun 
known  as  the  zodiacal  light. 

We  have  seen  that  devoted  students  of  comets  have  learned 
much  concerning  these  interesting  travelers.  Many  mysteries 
have  been  removed,  but  many  questions  remain  for  the  astrono- 
mers of  the  future  to  answer.  We  should  especially  like  to 
know  more  of  the  physical  conditions  existing  in  comets,  more 
about  their  chemical  contents,  and  more  as  to  why  and  how 
they  shine  by  their  own  light.  Perhaps  the  most  valuable 
result  of  cometary  investigation  has  been  the  emancipation  of 
civilized  peoples  from  unreasoning  and  groundless  fears  of 
these  bodies,  which  come  and  go  in  obedience  to  the  same 
simple  laws  that  govern  our  every-day  affairs. 


A  TOTAL  ECLIPSE  OF  THE  SUN1 

By  ROBERT  G.  AITKEN 

The  first  lecture  of  the  present  course  gave  a  general 
account  of  our  solar  system  as  a  whole,  emphasizing  par- 
ticularly the  harmonies  in  the  motions  of  its  component  bodies 
and  its  isolation  from  other  stellar  systems.  The  second 
lecture  described  in  detail  what  we  know  about  one  special 
class  of  objects  within  our  system — the  comets.  It  has  seemed 
to  me  appropriate  that  our  third  lecture  should  be  devoted  to 
the  Sun  itself,  the  most  important  object  in  the  universe  for 
us — the  source  of  heat,  light,  mechanical  and  electrical  power, 
and,  in  the  material  sense,  of  life  itself  on  our  little  globe. 

But  the  phenomena  of  the  Sun  as  revealed  by  our  modern 
studies  are  so  multifarious  and  raise  so  many  intricate  and 
interesting  problems  that  it  is  quite  impossible  to  treat  them 
all  in  a  single  lecture.  It  is  necessary  to  select,  and  I  have 
chosen  to  place  the  emphasis  in  what  I  shall  say  this  evening 
upon  those  phenomena  which  are  more  or  less  directly  asso- 
ciated with  a  total  eclipse  of  the  Sun. 

There  are  special  reasons  for  this  choice :  No  other 
natural  phenomenon  is  so  impressive,  so  startling,  so  fascinat- 
ing, as  a  total  eclipse  of  the  Sun ;  many  important  advances  in 
our  knowledge  of  the  Sun  have  had  their  origin  in  eclipse 
observations;  the  present  year  (1917)  is  a  year  of  eclipses — 
seven,  the  maximum  possible  number,  occurring  within  it ;  a 
total  eclipse  of  the  Sun  will  be  visible  in  the  western  part  of 
this  country  next  year — June  8,  1918 — for  the  first  time  in 
twenty-nine  years ;  and,  finally — a  point  of  particular  interest 
to  us  who  are  gathered  here — our  Society,  the  Astronomical 
Society  of  the  Pacific,  may  be  said  to  owe  its  existence  to  a 
total  eclipse  of  the  Sun.  This  was  the  eclipse  of  January  1, 
1889,  which,  beginning  at  sunrise  in  the  North  Pacific  Ocean, 
entered  California  near  Point  Arena  at  about  1 :45  p.  m.,  and 
swept  across  the  State  northeastwardly  in  a  path  some  eighty 
miles  broad,  to  end  at  sunset  in  northeastern  Canada. 


1  Delivered  January  12,  1917. 


A  TOTAL  ECLIPSE  OF  THE  SUN  53 

The  Lick  Observatory,  which  had  begun  active  work  only 
six  months  earlier,  sent  a  party  headed  by  the  late  Professor 
Keeler  to  a  favorable  station  on  the  central  line  of  the  shadow 
path.  Near  by  were  expeditions  from  other  American  observa- 
tories, and  a  strong  party  from  the  Amateur  Photographic 
Association  of  the  Pacific  Coast,  under  the  energetic  leader- 
ship of  Mr.  Charles  Burckhalter  of  the  Chabot  Observatory. 
This  party  of  amateurs  secured  many  very  successful  photo- 
graphs, which  were  later  discussed  by  Professor  Holden,  and 
the  results  published  in  Volume  I  of  the  Lick  Observatory 
Contributions.  It  was  the  cordial  cooperation  of  amateur  and 
professional  observers  on  this  occasion,  and  the  interest  in 
astronomy  revealed  and  stimulated  by  it  among  our  people, 
that  led  to  the  formation  of  our  Society. 

The  questions  which  I  think  you  would  like  to  have  me 
discuss  in  this  lecture  are :  ( 1 )  What  causes  an  eclipse  of  the 
Sun  or  of  the  Moon,  and  why  do  we  so  seldom  see  a  total 
eclipse  of  the  Sun?  (2)  What  do  astronomers  hope  to 
discover  at  the  time  of  a  total  eclipse  that  they  cannot  find  out 
by  studying  the  Sun  at  other  times  ?  (3)  Just  what  do  they  do 
to  get  ready  for  an  eclipse  and  during  the  few  minutes  of  its 
duration  ? 

It  does  not  require  a  vivid  imagination  to  picture  the 
terror  inspired  among  primitive  peoples  by  a  solar  eclipse.  To 
see  the  Sun  in  midday  slowly  but  surely  disappear  without 
apparent  cause  (for  the  Moon  is  quite  invisible  until  its  disk 
begins  to  encroach  upon  the  disk  of  the  Sun),  is  sufficiently 
awe-inspiring  even  to  those  who  understand  the  reason  and 
who  have  made  special  preparations  to  observe  the  phe- 
nomenon ;  and  it  is  easy  enough  to  see  how  such  myths  as  that 
of  the  dragon  devouring  the  Sun  came  into  being.  Even  in 
quite  modern  times  an  eclipse  of  the  Sun  was  seriously 
regarded  as  a  portent,  "a  sign  and  a  wonder  in  heaven/'  and 
there  is  a  quaint  story  concerning  a  total  eclipse  which  occurred 
in  our  own  colonial  days  while  the  General  Assembly  of 
Connecticut  was  in  session.  Many  members  were  alarmed, 
some  exclaimed  that  the  Judgment  Day  was  at  hand,  but  one 
sturdy  member  called  for  candles,  that  they  might  proceed 
with  their  business  and,  if  Judgment  Day  came,  be  found 
doing  their  duty. 


54  THE  ADOLFO  STAHL  LECTURES 

Long  before  the  dawn  of  recorded  history,  however,  far- 
seeing  men  like  the  Babylonian  and  Chaldean  watchers  of  the 
skies  had  learned  to  associate  eclipses  of  the  Sun  and  of  the 
Moon  with  the  motions  of  these  bodies  relatively  to  the  Earth, 
and  had  indeed  discovered  an  approximate  method  of  fore- 
casting eclipses  by  means  of  an  eclipse  cycle,  for  which  we 
still  use  the  name  they  gave — the  Saros. 

It  is  obvious  that  all  the  planets  and  satellites  in  our  system, 
since  they  shine  merely  by  reflected  sunlight,  must  constantly 
be  attended  by  shadows  sweeping  through  space  on  the  side 
turned  away  from  the  Sun,  and  that  these  shadows  must  be 
conical  in  shape  (since  the  bodies  casting  them  are  approxi- 
mately spheres),  with  bases  equal  to  the  cross-sections  of  the 
bodies  intercepting  the  Sun's  light,  and  lengths  depending 
upon  the  sizes  and  distances  of  these  bodies  from  the  Sun. 
Every  night  we  walk  in  the  Earth's  shadow,  and,  from  a 
mountain  height,  like  that  of  Mount  Hamilton,  or  from  the 
deck  of  a  ship  far  out  at  sea,  we  can  watch  that  shadow  sweep- 
ing up  the  eastern  sky  as  the  Sun  sinks  farther  and  farther 
below  the  western  horizon. 


FIG.  5.     SHADOW  AND  PENUMBRA  OF  EARTH  AND  MOON. 

A  marks  the  position  of  the  Moon  in  a  solar  eclipse, 
B,  in  a  lunar  eclipse.  An  eclipse  is  total  for  points  in 
the  shadow  cones,  partial  for  points  within  the  penum- 
brae. 

A  beautiful  example  of  such  a  shadow  is  that  afforded  by 
the  passage  of  one  of  Jupiter's  larger  satellites  across  the 
planet's  disk.  The  shadow  can  be  seen  by  our  telescopes  only 
when  it  falls  upon  the  planet,  and  then  it  appears  as  a  nearly 
round  black  dot  which  travels  across  the  bright  planet  from 
east  to  west  (Plate  II).  If  we  were  on  Jupiter  within  that 
shadow-spot,  the  Sun  would  be  eclipsed  for  us. 

Since  the  Moon  revolves  about  the  Earth  from  west  to 
east  once  every  month,  it  must  be  in  conjunction  (pass  between 


A  TOTAL  ECLIPSE  OF  THE  SUN  55 

the  Earth  and  Sun)  once  each  month — at  new  moon — and 
half  a  month  later  at  full  moon,  be  in  opposition — on  the 
opposite  side  of  the  Earth  from  the  Sun.  If  the  Moon's  orbit 
were  precisely  in  the  same  plane  as  that  of  the  Earth,  that  is, 
if  the  Moon's  apparent  path  among  the  stars  were  precisely 
the  same  as  that  of  the  Sun,  there  would  be  an  eclipse  of  the 
Sun  at  every  new  moon  and  one  of  the  Moon  at  every  full 
moon. 

If,  further,  the  Moon  and  Earth  were  perfect  spheres  and 
were  revolving  in  perfect  circles,  all  eclipses  of  the  Sun  would 
be  exactly  alike,  and  similarly  those  of  the  Moon.  As  a  matter 
of  fact,  none  of  these  conditions  is  realized,  and  no  two 
eclipses  are  quite  alike. 

The  Moon's  orbit  is  tilted  at  an  angle  of  about  5°  to  that 
of  the  Earth,  hence  it  generally  happens  that  the  shadow  of 
the  Moon  at  new  moon  passes  above  or  below  the  Earth,  and 
that  of  the  Earth  at  full  moon  above  or  below  the  Moon.  It  is 
only  when  the  Sun  at  new,  or  full  moon,  is  near  one  of  the 
lunar  nodes — the  name  we  give  to  the  two  points  where  the 
two  orbits  apparently  intersect — that  an  eclipse  can  occur.  An 
eclipse  of  the  Sun  must  happen  when  the  Sun  at  time  of  new 
moon  is  within  15%°  of  the  node,  and  may  happen,  under 
special  conditions,  when  it  is  as  far  as  18^°  from  the  node. 
The  limits  for  eclipses  of  the  Moon  are  somewhat  smaller. 
Now  since  the  Sun  appears  to  make  the  circuit  of  the  heavens 
once  each  year,  it  travels  less  than  30°  in  a  lunar  month. 
Hence  at  least  one  new  moon  must  occur  while  the  Sun  is 
still  within  15%°  of  the  node,  on  one  side  or  the  other,  and 
six  lunations  later  the  same  thing  must  happen  at  the  other 
node.  Therefore  there  must  be  at  least  two  eclipses  of  the 
Sun  each  year.  Because  the  limits  for  an  eclipse  of  the  Moon 
are  smaller,  it  occasionally  happens  that  a  year  will  pass  with- 
out any  lunar  eclipse. 

Suppose  a  total  eclipse  of  the  Moon  to  take  place  very  early 
in  the  year,  as  happened  this  year,  on  last  Sunday  night 
(January  7,  1917).  The  Moon  on  this  occasion  was  a  little 
west  of  its  descending  node,  and  the  Sun  near  the  opposite  or 
ascending  node.  Two  weeks  later,  at  new  moon  on  Monday, 
January  22,  the  Moon  has  overtaken  the  Sun  at  a  point  east  of 
the  descending  node  but  well  within  the  eclipse  limit,  and  a 


56  THE  ADOLFO  STAHL  LECTURES 

partial  eclipse  of  the  Sun  results.  Five  new  moons  after  this 
the  Sun  is  west  of  the  descending  node  and  within  the  eclipse 
limit,  giving  another  partial  solar  eclipse  on  June  18-19;  two 
weeks  later,  on  July  4,  it  is  close  to  this  node,  and  the  Moon, 
at  full,  is  near  the  ascending  node,  and  the  result  is  another 
total  eclipse  of  the  Moon;  two  weeks  later  still,  at  new  moon 
on  July  18,  the  Moon  has  overtaken  the  Sun  again  just  before 
it  reaches  the  eclipse  limit  east  of  the  descending  node,  and  a 
very  small  partial  solar  eclipse  takes  place, — three  eclipses 
within  a  month's  time.  Finally,  on  December  13,  Sun  and 
Moon  are  in  conjunction  so  near  the  ascending  node  that  an 
annular  eclipse  of  the  Sun  results.  Two  weeks  later,  on 
December  27,  comes  the  last  eclipse  of  the  year,  a  total  eclipse 
of  the  Moon — seven  eclipses  within  the  year.  This,  as  has 
been  said,  is  the  maximum  possible  number,  but  it  occasionally 
happens  that  five  out  of  the  seven  are  eclipses  of  the  Sun,  and 
only  two  of  the  Moon.  The  last  year  with  five  solar  eclipses 
was  1823  and  the  next  one  will  be  1935. 

I  have  mentioned  partial,  total  and  annular  eclipses  of  the 
Sun.  A  partial  eclipse  is,  of  course,  one  in  which  only  part  of 
the  Sun's  disk  is  covered  by  that  of  the  Moon,  and  needs  no 
comment  except  that  every  solar  eclipse  is  a  partial  one  for 
some  stations  on  the  Earth.  When  at  eclipse  time  the  line 
joining  the  centers  of  the  Sun  and  Moon  passes  through  any 
part  of  the  Earth  also,  which  happens  when  conjunction  takes 
place  within  10°  of  the  node,  the  eclipse  is  central.  If  the 
Moon's  shadow  reaches  the  Earth,  it  is  total,  viewed  from 
points  within  the  shadow  path ;  but  if  the  shadow  cannot  reach 
the  Earth  the  Moon's  disk  will  be  a  little  smaller  than  that  of 
the  Sun,  and  a  narrow  rim  or  annulus  of  sunlight  will  surround 
it  when  it  is  projected  on  the  Sun's  disk. 

The  actual  length  of  the  Moon's  shadow  and  the  distance 
of  the  Moon  from  the  Earth  are  continually  varying  because 
the  orbits  of  the  Earth  and  Moon  are  ellipses,  not  circles.  The 
following  table  gives,  in  round  numbers,  the  average,  the 
greatest  and  the  least  values  at  time  of  new  moon : 

Distance  from  Moon 

to  Earth's  Surface     Length  of  Moon's  Shadow       Difference 
Average    235,000  miles    Average  232,000  miles      -  3,000  miles 
Greatest    249,000  miles     Shortest  228,000  miles    —21,000  miles 
Least         218,000  miles     Longest  236,000  miles     +18,000  miles 


A  TOTAL  ECLIPSE  OF  THE  SUN  57 

It  follows  that  the  Moon's  shadow  cannot  always  reach 
the  Earth's  surface,  even  at  the  time  of  a  central  eclipse,  and 
that,  when  it  does,  the  cross-section  of  the  shadow  cone  where 
it  intersects  the  surface  may  vary  from  a  mere  point  to  a  circle 
about  168  miles  in  diameter.  Some  central  eclipses,  therefore, 
are  annular,  not  total,  and  a  total  eclipse  may  be  as  brief  as  a 
fraction  of  a  second  or  may  last  nearly  eight  minutes.  Eclipses 
lasting  as  long  as  six  minutes  are  the  exception,  however,  and 
the  majority  last  only  about  two  or  three  minutes. 

Another  point  should  be  noticed  while  we  are  discussing 
the  mechanism  of  eclipses.  The  Moon  revolves  about  the 
Earth  from  west  to  east,  hence  the  Moon's  shadow  at  the  time 
of  a  total  eclipse  always  touches  the  Earth  first  at  a  point  where 
the  Sun  is  just  rising,  sweeps  on  eastwardly  and  leaves  the 
Earth  at  a  point  where  the  Sun  is  just  setting.  Meanwhile  the 
Earth  itself  is  turning  on  its  axis  from  west  to  east,  thus 
shortening  the  path  along  which  the  shadow  travels  to  about 
120°  of  longitude.  Further,  the  Earth  rotates  on  an  axis 
perpendicular  to  the  equator,  and  the  angle  between  the 
planes  of  the  equator  and  the  ecliptic  is  about  23^2°.  Hence, 
since  the  plane  of  the  Moon's  orbit  makes  an  angle  of  5°  with 
that  of  the  ecliptic,  we  see  that  the  Moon  at  the  node  is  moving 
sometimes  at  an  angle  of  more  than  28°  to  the  equator,  some- 
times at  one  only  a  little  over  18°,  and  this  motion  will  be 
toward  the  north  at  one  node  and  toward  the  south  at  the  other. 
The  shadow  path  on  the  Earth's  surface  at  eclipse  time  -is 
therefore  a  curve  tending  in  a  general  northeasterly  or  south- 
easterly direction,  the  actual  figure  depending  upon  the  angle 
between  the  Moon's  orbit  and  the  equator,  and  the  latitude  in 
which  the  eclipse  occurs  (Fig.  6). 

Any  given  total  eclipse  is  visible  as  such  only  from  stations 
in  the  comparatively  narrow  shadow  path — ordinarily  less  than 
100  miles  wide — and  in  general  this  path  crosses  any  given 
spot  on  the  Earth's  surface  only  at  long  intervals.  For  ex- 
ample, the  last  total  eclipse  visible  from  points  in  the  British 
Islands  occurred  in  1724,  the  next  one  will  not  take  place  until 
1927.  The  area  within  which  the  eclipse  is  visible  as  partial  is, 
of  course,  much  wider,  extending  indeed  several  thousand 
miles  on  either  side  of  the  shadow  track. 


58 


THE  ADOLFO  STAHL  LECTURES 


I  have  said  that  the  ancients  had  discovered  that  an  eclipse 
returns  after  a  period  of  about  eighteen  years,  a  period  to 
which  they  had  given  the  name  Saros.  In  one  sense  every 
eclipse  is  the  return  of  its  predecessor,  but  in  another  sense 
the  statement  just  made  is  appropriate  and  the  Saros  is  a  cycle 
of  considerable  interest. 


FIG.  6.    TOTAL  ECLIPSE  OF  JUNE  8,  1918. 

The  nearly  parallel  lines  across  the  center  mark  the  shadow  path ;  the 
longer,  closed  curves  indicate  within  what  wide  limits  the  eclipse  was 
visible  as  partial. 

The  Moon  makes  the  circuit  from  new  moon  back  to  new 
in  what  we  call  our  ordinary  month,  29.53059  days,  but  it 
requires  only  27.21222  days — a  draconic  month — to  pass  from 
node  around  to  the  same  node  again  because  the  nodal  points 
are  constantly  retrograding,  slipping  westward  along  the 
ecliptic,  an  effect  due  to  what  we  call  perturbing  forces  that 
we  cannot  stop  to  discuss  tonight.  For  the  same  reason — this 
retrogression  of  the  nodes — the  Sun  passes  from  a  node  around 
to  the  same  node  in  346.6201  days  instead  of  in  365^4  days. 


A  TOTAL  ECLIPSE  OF  THE  SUN  59 

Now  let  us  multiply  the  first  period  by  223,  the  second  by  242, 
the  third  by  19.  We  shall  have,  with  sufficient  accuracy  for 
our  purpose,  6,585.32,  6,585.35  and  6,585.78  days  respectively, 
and  these  values  amount  to  eighteen  years,  eleven  days  (ten 
days  if  five  leap  years  are  included)  and  the  three  fractions 
given.  After  this  interval,  which  is  known  as  the  Saros,  the 
three  bodies  will  again  stand  almost  precisely  in  the  same  rela- 
tion to  each  other,  and  if  an  eclipse  takes  place  at  a  given  date, 
one  Saros  later  another  will  occur  under  almost  the  same  con- 
ditions. Almost,  not  quite.  Because  of  those  three  differing 
fractions  of  a  day,  the  Sun  and  Moon  will  be  a  little  farther  west 
with  respect  to  the  node  at  the  second  eclipse,  causing  slight 
changes  in  the  direction  and  length  of  the  shadow,  and  the 
Earth  will  have  turned  nearly  one-third  way  farther  round  on 
its  axis,  causing  the  center  of  the  second  eclipse  to  fall  cor- 
respondingly farther  west  on  its  surface.  This  second  eclipse 
we  consider  as  a  "return"  of  the  earlier  one,  for  though  many 
others  have  taken  place  between  the  two,  the  positions  of  Sun 
and  Moon  with  respect  to  the  node,  and  hence  the  circum- 
stances of  these  eclipses,  were  quite  different. 

To  see  how  this  cycle  may  be  used  in  making  approximate 
forecasts  of  eclipses,  let  us  compare  the  eclipses  which  occurred 
one  Saros  ago  with  those  which  are  taking  place  in  the 
present  year : 

(1)  Total  eclipse  of  Moon,  1898,  December  27,  1917,  January  7. 
(Duration   of  totality,  1^29™                            Ih28m) 

(2)  Partial  eclipse  of  Sun,  1899,  January  11.  1917,  January  22. 
(Magnitude  of  eclipse,  0.715                            0.725)" 

(3)  Partial  eclipse  of  Sun,  1899,  June  7.  1917,  June  18-19. 
(Magnitude  of  eclipse,  0.608                            0.473) 

(4)  Total  eclipse  of  Moon,  1899,  June  22-23.  1917,  July  4. 
(Duration  of  totality,  Ih1.5m                           lhf.5m) 

(5)  Partial  eclipse  of  Sun,  1917,  July  18. 

(Magnitude,  0.086) 

(6)  Annular  eclipse  of  Sun,     1899,  December  2.       1917,  December  13. 
(Center  of  each  eclipse  track  near  the  South  Pole.) 

(7)  Eclipse  of  Moon,  1899,  December  16.     1917,  December  27. 
(Almost   total   in    1899,   magnitude  =  0.996 ;   just   total   in    1917, 

magnitude  =  1.011,,  duration  of  totality  =  16. 5m.) 

Each  lunar  eclipse  of  the  earlier  period,  it  is  seen,  is 
repeated  this  year,  the  date  falling  eleven  days  later,  the 
duration  of  totality  being  about  the  same.  The  three  solar 


60  THE  ADOLFO  STAHL  LECTURES 

eclipses  of  the  earlier  year  are  followed  this  year,  eleven  days 
later  in  the  year,  by  eclipses  resembling  them  closely,  the 
point  of  greatest  eclipse,  however,  falling  this  year  about  120° 
of  longitude  farther  west.  In  addition,  a  new  cycle  begins 
this  year  with  the  very  small  partial  eclipse  of  the  Sun  on 
July  18. 

Let  us  illustrate  the  recurrence  of  a  single  eclipse  at  Saros 
intervals  by  considering  the  cycle  to  which  the  eclipse  of 
June  8,  1918,  belongs.  Like  all  eclipse  cycles  this  one  began 
as  a  very  slight  partial  eclipse,  when  the  Sun  was  almost  at 
the  limit  of  distance  east  of  the  Moon's  node  at  the  time  of  new 
moon.  Since,  for  this  family  of  eclipses,  the  new  moon  was 
near  the  ascending  node,  the  point  where  its  orbit  crosses  the 
ecliptic  from  south  to  north,  the  penumbra  brushed  the  Earth 
near  the  South  Pole  at  this  first  eclipse — on  March  10,  1179. 
Eighteen  years  later,  on  March  20,  1197,  the  Sun  was  a  little 
nearer  the  node  at  new  moon,  and  the  Moon's  disk  cut  off  a 
little  more  of  the  Sun's  light.  This  continued  after  every 
Saros,  the  magnitude  of  the  eclipse  increasing  each  time,  until 
June  4,  1323,  when  the  eclipse  became  central  and  annular  for 
a  short  track  near  the  Earth's  South  Pole.  Annular  eclipses 
continued  after  each  succeeding  Saros,  twenty-eight  of  them, 
their  paths  falling  ever  farther  north,  until  April  14,  1828. 
By  this  time  the  Sun,  at  new  moon,  had  passed  the  node,  and 
at  the  same  time  the  Moon  was  nearer  perigee  (the  point  in  its 
orbit  nearest  the  Earth)  and  hence  the  tip  of  the  actual  shadow 
cone  touched  the  Earth  at  the  middle  of  the  eclipse  time  at  a 
station  in  East  Africa,  18°  north  of  the  equator,  completely 
hiding  the  Sun  there  for  a  few  seconds.  The  conditions  at  the 
next  return  were  similar,  the  eclipse  in  April  25,  1846,  being 
annular  along  the  eclipse  track  except  just  east  of  Cuba,  in  25° 
north  latitude,  where  it  was  total.  The  three  following  returns, 
May  6,  1864,  May  17,  1882,  May  28,  1900,  were  total,  the 
shadow  paths  spiraling  ever  northward.  The  shadow  cone  on 
June  8,  1918,  will  touch  the  Earth  at  sunrise  in  the  Pacific 
Ocean  in  130°  east  longitude  and  26°  north  latitude,  at  noon 
will  cross  a  point  in  the  Pacific  at  152°  west  longitude  and  51° 
north  latitude,  will  enter  the  United  States  in  southwestern 
Washington  at  about  2h  55m,  Pacific  Standard  Time,  sweep  a 
path  across  the  country  toward  the  southeast,  tapering  in 


PLATE  XIII.    THE  40-Foox  CAMERA,  FLINT  ISLAND. 


A  TOTAL  ECLIPSE  OF  THE  SUN  61 

width  from  a  little  over  seventy  miles  in  Washington  to  less 
than  forty-five  miles  in  Florida,  and  end  at  sunset,  in  the 
Atlantic  east  of  Cuba,  in  west  longitude  75°,  north  latitude  25°. 

At  subsequent  returns  it  will  spiral  ever  farther  north, 
remaining  total  until  the  return  of  August  23,  2044.  Then  for 
more  than  two  hundred  years  it  will  recur  as  a  partial  eclipse, 
disappearing  at  length  above  the  North  Pole  when  the  Sun 
at  new  moon  has  passed  beyond  the  eclipse  limit  west  of  the 
node. 

The  actual  calculation  of  an  eclipse  path  and  the  other 
attending  circumstances,  such,  for  example,  as  the  precise  time 
and  duration  of  totality  at  a  given  station  within  the  path,  are 
far  too  technical  matters  to  be  dealt  with  tonight.  Suffice  it  to 
say  that  the  calculation  can  be  made  with  such  accuracy  that 
we  might  easily  select  now  a  station  for  the  eclipse,  say  of 
August  23,  2044,  and  set  up  our  telescopes  there  with  the  full 
assurance  that  the  eclipse  would  occur  within  a  few  seconds 
of  the  predicted  time  and  that  our  telescopes  would  need  but 
very  slight  adjustments  by  the  observers  of  that  distant  day. 
Whether  they  would  succeed  in  making  the  observations 
planned  is  another  matter.  Perhaps  by  that  time  our  successors 
will  have  learned  how  to  control  the  Earth's  atmosphere  so  as 
to  insure  a  clear  sky  at  the  critical  moments.  At  present  we 
can  not  do  this,  and  therefore  the  intending  observer,  in 
selecting  a  station  for  his  observations,  carefully  studies  all  the 
meteorological  data  available,  and  when  the  path  of  totality 
makes  choice  of  stations  possible,  gives  the  meteorological 
factor  almost  the  highest  weight.  Of  course,  he  desires  a 
position  where  the  eclipse  will  be  of  maximum  duration,  for 
at  best  it  is  all  too  short,  but  if  weather  conditions  there  are 
highly  unfavorable,  and  are  far  more  promising  at  a  point 
where  the  eclipse  time  is  shorter,  the  latter  will  be  preferred. 
Often  it  happens  that  the  shadow  path  lies  mainly  across  the 
ocean,  touching  land  only  at  the  edges  of  the  continents  near 
sunrise  and  sunset,  and  perhaps  crossing  an  island  or  two.  It 
may  easily  happen  that  at  every  possible  station  at  such  an 
eclipse  the  chances  for  clouds  are  so  great  that  no  observer 
will  care  to  risk  the  time  and  expense  an  expedition  thither 
would  involve. 


62  THE  ADOLFO  STAHL  LECTURES 

Unfortunately  an  accurate  forecast  of  the  state  of  the 
atmosphere  at  eclipse  time  is  impossible  even  at  the  most 
promising  station,  a  fact  that  laymen  sometimes  find  it  hard 
to  understand.  For  example,  our  eclipse  observers  returning 
from  the  eclipse  of  May  28,  1900,  in  Georgia,  recounted  with 
glee  the  skepticism  of  a  leading  citizen  of  a  small  town  there 
who  had  from  the  first  been  doubtful  of  their  ability  to  foretell 
the  occurrence  of  the  eclipse.  When  he  heard  their  anxious 
discussions  as  to  the  probabilities  -of  cloudiness  at  the 
important  time,  his  doubt  was  deepened  to  conviction.  "These 
young  men  try  to  tell  me  they  know  the  Sun  is  going  to  be 
eclipsed  and  they  can't  even  fell  me  if  the  sky  is  going  to  be 
clear!" 

Better-informed  people  may  well  ask,  since  the  duration 
of  an  eclipse  is  so  short  and  the  chances  of  observing  it  are 
at  best  uncertain,  why  astronomers  should  devote  weeks  and 
months  of  time  to  preparation,  and  travel  sometimes  half 
around  the  world  to  watch  the  phenomenon.  Let  me  answer 
by  tracing  in  a  summary  way  the  development  of  our  knowl- 
edge of  the  Sun  during  the  last  eighty  years.  It  is  not  neces- 
sary to  go  back  farther,  for,  broadly  speaking,  we  may  say 
that  little  more  was  really  known  about  the  Sun  in  1840  than 
had  been  discovered  by  Galileo  and  his  contemporaries  and 
their  immediate  successors  in  the  early  days  of  the  telescope 
two  centuries  before. 

The  Sun  was  an  enormous  globe  whose  composition  and 
physical  condition  were  unknown.  From  its  intensely  hot  sur- 
face, known  as  the  photosphere,  light  and  heat  were  radiated. 
From  time  to  time  spots  appeared  on  this  surface  and  by 
observation  it  was  found  that  they  were  confined  to  two  broad 
zones,  one  on  either  side  of  the  Sun's  equator,  that  they  were 
often  surrounded  by  areas  of  extreme  brightness — the  faculse 
— and  that  the  Sun  turned  on  its  axis  once  in  twenty-five  or 
twenty-six  days.  Total  eclipses  of  the  Sun  had  been  observed 
when  the  shadow  paths  were  conveniently  placed,  but  princi- 
pally to  note  the  precise  times  of  contact  of  the  disks  of  the 
Sun  and  Moon  for  the  purpose  of  improving  the  lunar  and 
solar  tables.  The  corona  was  noted  of  course,  it  could  hardly 
have  escaped  the  notice  of  even  the  earliest  witnesses  of  an 
eclipse,  and  there  are  occasional  references  to  rosy  or  scarlet 


A  TOTAL  ECLIPSE  OF  THE  SUN  63 

or  flame-colored  appearances  close  to  the  Moon's  disk  during 
totality,  but  these  features  attracted  strangely  little  scientific 
attention. 

One  discovery  of  capital  importance  had  been  made,  though 
not  at  an  eclipse.  Fraunhofer,  in  1815,  had  found  that  the 
solar  spectrum,  produced  by  passing  a  beam  of  sunlight  through 
a  narrow  aperture  or  slit  and  then  through  a  prism,  is  crossed 
by  a  series  of  fine  dark  lines  which  always  fall  in  the  same 
positions  with  respect  to  the  colors  of  the  spectrum,  but  their 
significance  was  unknown. 

The  real  impetus  to  further  advance  in  solar  studies,  we 
may  say,  was  given  by  the  eclipse  of  July  8,  1842.  The  Moon's 
shadow  on  that  occasion  swept  across  Europe,  and  many 
prominent  astronomers  occupied  stations  on  the  shadow  path. 
The  corona  was  strikingly  beautiful  and,  fortunately,  at  least 
three  large  brilliant  flame-colored  protuberances,  now  known 
as  prominences,  were  visible.  What  caused  them,  and  what 
was  the  corona?  These  questions  were  now  for  the  first  time 
generally  discussed,  and  it  was  soon  apparent  that  astronomers 
were  divided  in  their  opinions.  Some  held  that  they  were 
solar  appendages,  others  that  they  belonged  to  the  Moon, 
while  a  third  group  argued  that  they  were  not  objective 
realities  at  all,  but  were  optical  phenomena  produced  by 
diffraction  of  the  Sun's  light  at  the  irregular  mountainous 
circumference  of  the  Moon's  disk.  Eclipses  of  the  Sun  were 
now  looked  forward  to  with  interest,  and  in  the  next  thirty 
years  a  number  occurred  that  were  well-  observed.  Moreover, 
new  instruments  were  made  available  to  study  their  phenomena. 

Photographic  processes  had  been  so  far  perfected  that  they 
could  be  systematically  applied  at  the  Spanish  eclipse  of 
July  18,  1860.  In  the  preceding  year,  1859,  Kirchhoff  had 
shown  that  the  Fraunhofer  lines  could  be  explained  on  the 
assumption  that  the  light  from  the  Sun's  photosphere  passes 
through  a  gaseous  layer  or  envelope  which,  while  intensely 
hot,  is  cooler  than  the  photosphere  itself.  This  layer  of  gases 
would  "absorb"  light  of  precisely  the  wave-lengths  it  was  itself 
capable  of  emitting.  Hence  the  positions  of  the  lines  should 
not  only  tell  us  the  composition  of  the  gaseous  layer,  but  when 
the  photospheric  light  is  cut  off,  as  for  example,  by  the  inter- 
position of  the  Moon's  disk  at  the  time  of  total  eclipse,  the  lines 


64  THE  ADOLFO  STAHL  LECTURES 

themselves  should  flash  out  as  bright  lines.  Precisely  this 
phenomenon  was  observed  by  C.  A.  Young  at  the  eclipse  of 
December  22,  1870.  He  was  watching  the  Fraunhofer  lines  in 
his  spectroscope  as  the  Sun  gradually  disappeared  behind  the 
Moon's  advancing  disk,  and  just  as  the  last  rays  of  photo- 
spheric  light  were  cut  off  he  saw  them  suddenly  flash  out  as 
bright  lines.  In  a  second  or  two  they  were  gone — covered  by 
the  advancing  Moon.  But  the  existence  of  the  "reversing 
layer"  above  the  photosphere  had  been  fully  established  by 
this  actual  observation  of  the  "flash  spectrum." 

Meanwhile  the  photographic  camera  and  spectroscope  had 
definitely  proved :  ( 1 )  that  the  prominences  and  the  inner 
corona  were  real  and  belonged  to  the  Sun,  for  the  Moon's 
disk  clearly  traversed  them  in  its  motion;  (2)  that  the  promi- 
nences were  vast  masses  of  luminous  gases — hydrogen,  helium, 
calcium — rising  from  a  continuous  layer  (the  chromosphere) 
of  such  materials  surrounding  the  Sun;  (3)  that  the  corona 
was  at  least  partly  gaseous,  for  its  spectrum  showed  a  bright 
line  of  green  light  due  to  some  element  not  even  yet  identified 
but  called  "coronium" ;  but  (4)  that  it  shone  also  in  part  by 
reflected  sunlight,  for  Fraunhofer  lines  were  present,  and  the 
light  was  partly  polarized. 

If  an  astronomer  had  been  fortunate  enough  to  observe 
successfully  every  total  eclipse  that  has  occurred  from  1860 
to  the  present  year,  he  would,  in  all,  have  had  less  than  two 
hours'  time  of  actual  observation,  yet  it  is  clear  that  this  short 
space  of  observing  time  has  advanced  our  knowledge  of  the 
Sun  beyond  the  dreams  of  astronomers  a  century  ago.2 

Total  eclipses  of  the  Sun  must  still  play  their  part  in  ad- 
vancing our  knowledge  of  the  forces  that  are  in  action  upon 
the  Sun,  and  of  the  relations  between  corona,  prominences  and 
sun-spots,  though  we  may  not  now  hope  to  discover  new 
enveloping  layers.  The  corona  has  been  seen  and  photographed 
only  at  the  time  of  total  eclipse,  in  spite  of  strenuous  efforts 
made  by  the  most  skilful  observers,  and  it  now  seems  that  the 
attempt  to  study  it  at  other  times  is  hopeless.  For  Abbot, 
using  that  extremely  delicate  electric  thermometer  which  we 

2  A  general  description  of  the  Sun  as  we  know  it  was  given  at  this  point  in 
the  spoken  lecture;  this  is  here  omitted,  for  a  better  account  will  be  found  in 
Dr.  St.  John's  lecture  on  a  later  page. 


A  TOTAL  ECLIPSE  OF  THE  SUN 


65 


call  the  bolometer,  an  instrument  that  can  reveal  the  variation 
of  0.000,000,1°  C.  of  heat  radiation,  has  shown  that  the  sky 
radiation  even  20°  from  the  Sun  is  more  than  ten  times  greater 
than  that  of  even  the  bright  inner  corona,  and  that  the  latter 
is  therefore  beyond  the  reach  of  any  existing  form  of  instru- 
ment except  at  times  of  eclipse. 

Eclipses,  too,  afford  the  best  if  not  the  only  opportunity  to 
study  other  questions  not  strictly  related  to  the  constitution  of 
the  Sun ;  for  example,  whether  or  not  there  exists  a  planet  of 
any  notable  size  within  the  orbit  of  Mercury,  and  whether  the 
force  of  gravity  has  the  power  to  deflect  light,  as  postulated 
by  the  modern  theory  of  relativity.  The  former  question  was 
prominent  in  the  plans  of  recent  eclipses  but  has  now  been 
quite  definitely  settled  in  the  negative,  mainly  by  the  observa- 
tions by  Lick  Observatory  expeditions.  The  letter  will 
certainly  hold  a  prominent  place  in  the  program  for  the  eclipse 
of  June  8,  1918. 

Thanks  to  the  liberality  of  generous  friends — the  late 
Colonel  Fred  Crocker,  Mrs.  Phebe  A.  Hearst,  and,  particu- 
larly, Mr.  W.  H.  Crocker,  all  members  of  our  Society — the 
Lick  Observatory  has  from  the  first  been  able  to  take  a 
prominent  part  in  solar  eclipse  work.  Since  the  California 
eclipse  of  January  1,  1889,  of  which  I  spoke  at  the  beginning, 
eclipse  expeditions  have  been  sent  out  at  the  expense  of  one 
or  another  of  these  three  friends  of  the  observatory  in  nine 
different  years,  and  in  only  two  of  these  years — 1896  and  1914 
did  clouds  prevent  success.3  Other  eclipses  have  occurred 

3  Eclipse  Expeditions  from  the  Lick  Observatory,   University  of  California. 


Date 

Place 

Donor 

In  Charge  of 

1889,  Jan.       1 

Bartlett   Springs,   Cal. 

The  University 

Keeler 

1889,  Dec.     22 

Cayenne,    French 

Chas.   F.   Crocker 

Burnham    & 

Guiana 

Schaeberle 

1893,   Apr.      16 

Mina  Bronces,   Chile 

Mrs.  Phebe  A.  Hearst 

Schaeberle 

1896,  Aug.       9 

Yezo,  Japan 

Chas.  F.   Crocker 

Schaeberle   (clouds) 

1898,  Jan.      22 

Jeur,   India 

Chas.   F.   Crocker 

Campbell 

1900,  May     28 

Thomaston,   Ga. 

W.  H.   Crocker 

Campbell 

1901,  May     18 

Padang,    Sumatra 

W.  H.   Crocker 

Perrine 

1905,  Aug.     20 

Cartwright,    Labrador 

W.  H.   Crocker 

Curtis   (clouds) 

1905,  Aug.     20 

Alhama,   Spain 

W.   H.   Crocker 

Campbell 

1905,  Aug.     20 

Aswan,    Egypt 

W.  H.   Crocker 

Hussey 

1908,  Jan.        3 

Flint  Island, 

W.  H.   Crocker 

Campbell 

South    Pacific 

1914,   Aug.     21 

Brovary,    Russia 

W.  H.  Crocker 

Campbell   &   Curtis 

(clouds) 

1918,  June      8 

'Goldendale,    Wash. 

W.  H.   Crocker 

Campbell  &  Curtis 

66  THE  ADOLFO  STAHL  LECTURES 

within  this  period,  but  the  prospects  of  good  weather  were  too 
poor  to  justify  an  expedition.  Doubtless  a  party  will  be  sent 
from  the  Lick  Observatory4  in  June,  1918,  to  a  suitable  sta- 
tion— perhaps  in  southern  Idaho  or  eastern  Oregon,  where 
weather  conditions  are  unusually  promising  and  where  the  total 
phase  on  June  8  will  last  a  little  less  than  two  minutes. 

Let  me  illustrate  what  such  an  expedition  means,  especially 
when  the  site  chosen  is  out  of  the  regular  lines  of  travel,  by 
giving  some  details  of  the  expedition  from  the  Lick  Observa- 
tory, which  it  was  my  privilege  to  accompany,  to  observe  the 
eclipse  of  January  3,  1908. 

The  Moon's  shadow  on  that  date  touched  the  Earth  at 
sunrise  in  the  Pacific  Ocean  in  longitude  155°  east,  latitude 
11°  north,  swept  eastward  and  left  the  Earth  at  sunset  on  the 
western  coast  line  of  Costa  Rica.  Two  small  islands  were  the 
only  land  points  in  the  shadow  path,  both  out  of  the  usual  lines 
of  steamer  travel.  There  was  no  choice  between  them,  either 
in  point  of  climate  or  in  point  of  accessibility,  or,  better,  inac- 
cessibility ;  but  at  Flint  Island,  about  450  miles  northwest  of 
Tahiti,  in  10°  7'  west  longitude,  11°  25'  south  latitude,  the 
eclipse  occurred  nearer  noon,  and  hence  when  the  Sun  'stood 
higher  in  the  sky  (an  advantageous  factor)  and  the  total  phase 
lasted  considerably  longer  than  at  Hull  Island,  which  is  about 
700  miles  north  of  Samoa,  Flint  Island  was  therefore  selected 
as  our  station. 

Mr.  W.  H.  Crocker  made  generous  provision  for  the 
expedition,  the  U.  S.  Navy  Department  courteously  detailed 
a  gunboat,  the  Annapolis,  to  take  the  party  from  Papeete, 
Tahiti,  the  nearest  steamer  port,  to  the  island  and  back ;  and 
plans  for  the  instrumental  equipment  and  for  the  observing 
program  were  begun  more  than  a  year  in  advance  of  the  date 
of  the  eclipse. 


*  Note  added  November,  1918.  The  Crocker  Eclipse  Expedition  from  the 
Lick  Observatory  occupied  a  station  at  Goldendale,  Washington,  on  June  8,  1918, 
and  added  another  to  the  Observatory's  list  of  successfully  observed  eclipses.  The 
circumstances  were  even  more  dramatic  than  at  the  eclipse  on  Flint  Island  which 
I  have  described  in  the  lecture. 

"The  sky  had  clouded  late  on  the  night  of  the  7th,"  writes  Dr.  Campbell, 
"and  we  may  say  that  it  remained  completely  clouded  until  toward  midnight  of  the 
8th,  with  the  important  exception  that  a  small  rift  occurred  exactly  at  the  critical 
time  and  place.  The  clouds  uncovered  the  Sun  less  than  one  minute  before  the 
beginning  of  totality  and  they  again  covered  the  Sun  a  few  seconds  after  the  end 
of  totality.  The  small  region  of  unclouded  sky  containing  the  totally  eclipsed  Sun 
seemed  to  be  absolutely  clear  .  .  ."! 

The  observing  program,  which  conformed  closely  to  the  forecast  made  in  the 


FIG.  1 — The  channel  through  the  reef. 


FIG.  2 — Surf-boat  used  in  landing  eclipse  equipment. 


FIG.  3 — Palm-thatched  huts  in  the  cocoanut  grove. 
PLATE  XIV.     FLINT  ISLAND. 


A  TOTAL  ECLIPSE  OF  THE  SUN  67 

Ever  since  the  eclipse  of  1893,  the  chief  photographic  tele- 
scope used  on  our  expeditions  has  been  of  the  tower  form 
devised  and  used  at  that  time  by  Astronomer  Schaeberle  of  the 
Lick  Observatory,  at  Mina  Bronces,  Chile.  Obviously  it  is  not 
possible  to  transport  to  eclipse  stations  such  massive  instru- 
ments as  those  used  in  fixed  observatories  at  the  present  day ; 
and  the  small  portable  equatorials  used  at  eclipses  before  1893 
give  images  of  the  Sun  too  small  to  permit  satisfactory  study 
of  the  finer  details  of  the  coronal  structure.  What  is  wanted 
is  an  instrument  of  very  long  focus  which  will  give  a  solar 
image  four  or  more  inches  in  diameter,  and  which  will,  at  the 
same  time,  be  easy  to  transport  and  erect.  These  requirements 
the  Schaeberle  form  of  telescope,  especially  as  improved  in  its 
mounting  by  Campbell,  meets  admirably. 

Its  essential  parts  are  a  lens  of  40-foot  focal  length,  giving 
an  image  of  the  Sun  4^  inches  in  diameter ;  a  tube  consisting 
of  a  frame-work  of  lengths  of  gas-pipe,  screwed  together  and 
braced  by  stout  wire,  and  a  cover  of  black  cloth;  and  a  plate- 
holder  moved  by  clockwork.  The  geographical  coordinates  of 
the  station  being  known,  we  can  compute  the  precise  position  in 
the  sky  which  the  Sun  will  occupy  at  the  time  of  totality,  and 
we  can  therefore  mount  the  lens  rigidly  at  the  top  of  a  suitable 
tower  in  such  manner  that  at  the  time  of  eclipse  the  light  from 
the  corona  will  shine  down  the  tube  and  fall  centrally  upon  a 
photographic  plate  exposed  in  a  dark-room  at  its  lower  end.  The 
tube  is  not  attached  either  to  the  lens  or  to  the  photographic 
plate  but  simply  serves  to  connect  the  two  in  order  to  keep  stray 
light  from  falling  upon  the  plate.  The  lens  is  firmly  fixed  in 
position  in  advance  and  it  must  be  adjusted  with  precision. 
If  it  is  not  properly  set  it  cannot  be  changed  at  the  instant  of 
eclipse ;  if  such  mistake  should  be  made  the  telescope  would  be 
useless.  Since  the  Sun's  image  changes  its  position  slightly 

closing  paragraphs  of  my  lecture,  except  that  no  spectroheliograph  was  used  and 
that  no  "moving  picture"  record  was  secured,  was  carried  through  with  excellent 
results.  In  particular,  the  photographs  of  the  corona  are  the  finest  and  most 
interesting  ones  taken  at  any  Lick  Observatory  Crocker  Eclipse  Expedition.  One 
of  these  is  reproduced  in  half-tone  as  the  frontispiece  to  the  present  volume.  It 
is  hardly  necessary  to  say  that  much  of  the  delicate  detail  of  the  coronal  struc- 
ture shown  on  the  original  negative  has  been  lost  in  the  process  of  reproduction. 
It  is  impossible  as  yet  to  say  what  value  the  photographs  taken  to  test  the 
Einstein  theory  of  relativity  may  have,  because  circumstances  beyond  the  Observa- 
tory's control  prevented  the  taking  in  advance  of  check  photographs  of  the  region 
of  the  sky  in  which  the  Sun  would  stand  at  the  time  of  eclipse.  The  apparatus 
will  be  set  up  at  Mt.  Hamilton  early  this  winter,  when  this  region  is  again  visible 
at  night,  and  the  necessary  photographs  will  then  be  secured. 


68  THE  ADOLFO  STAHL  LECTURES 

during  the  minutes  of  the  total  phase,  because  of  the  Earth's 
revolution  about  the  Sun,  it  is  necessary  to  move  the  plate  at 
the  same  rate.  This  is  accomplished  by  attaching  a  simple 
driving  mechanism,  actuated  by  clockwork,  to  the  plate-holder. 
The  whole  apparatus  can  be  compactly  packed  and  can  be 
erected  with  little  difficulty. 

On  the  expedition  to  Flint  Island  an  instrument  of  this 
type  was  taken  to  secure  large-scale  images  of  the  corona. 
Another  Jens  of  comparatively  short  focus,  and  differently 
mounted,  was  taken  to  secure  additional  photographs,  especially 
of  the  outer  extensions  of  the  corona.  Spectrographs  of 
several  different  forms  were  designed  and  built  to  record  the 
general  spectrum  of  the  corona,  the  spectra  of  the  prominences, 
and  the  "flash  spectrum".  The  equipment  also  included  pho- 
tometers to  measure  the  intensity  of  the  coronal  light  at  varying 
distances  from  the  Moon's  apparent  limb,  polarigraphs  to  test 
its  polarization,  a  duplicate  set  of  telescopes — four  in  each  set — 
to  photograph  the  region  about  the  Sun  for  the  purpose  of 
detecting  an  intra-Mercurial  planet,  if  such  existed  (though 
earlier  expeditions  had  made  such  existence  doubtful),  and  an 
alt-azimuth  instrument  to  determine  time,  latitude  and  longi- 
tude. 

All  of  these  instruments  except  the  40-foot  telescope  and 
the  alt-azimuth  instrument  had  to  be  mounted  on  polar  axes 
driven  by  clockwork,  that  the  light  from  the  corona  might  pass 
through  the  various  optical  trains  and  fall  continuously  upon 
the  same  points  of  the  photographic  plates  during  the  exposure 
times.  In  regular  observatories  such  axes  are  of  metal  and  are 
very  heavy ;  for  our  eclipse  mountings  we  fit  metal  bearings 
in  the  ends  of  the  stout  wooden  packing  boxes  in  which  the 
instruments  themselves  are  transported.  These  boxes  then 
serve  as  polar  axes ;  they  are  supported  by  wooden  tripods,  and 
the  Spectrographs  or  other  pieces  of  apparatus  are  attached  to 
their  sides  and  they  are  rotated  by  means  of  long  lever  arms 
weighted  with  stone  or  sand,  the  rate  of  fall  being  controlled  by 
a  simple  clock. 

Every  instrument  was  set  up  at  Mount  Hamilton,  carefully 
adjusted,  and  fully  tested  before  being  packed.  Moreover,  the 
sky  area  in  which  the  Sun  would  be  at  the  time  of  eclipse  was 


A  TOTAL  ECLIPSE  OF  THE  SUN  69 

photographed  with  the  intra-Mercurial  camera  to  furnish 
comparison  plates  which  would  show  all  of  the  stars  in  the 
area. 

In  addition  to  the  larger  instruments,  smaller  pieces,  chro- 
nometers, driving-clocks,  tools,  developing  trays,  chemicals, 
lanterns  and  a  multitude  of  miscellaneous  materials  had  to  be 
provided.  It  was  also  known  that  the  island  was  small  and 
produced  only  cocoanuts,  and  experience  on  earlier  expeditions 
had  shown  the  desirability,  in  any  event,  of  making  the  observ- 
ing party  while  at  the  station  as  independent  as  possible  of  the 
country.  It  was  therefore  necessary  to  include  camp  cots, 
bedding,  kitchen  utensils,  dishes,  and  food  supplies  sufficient  to 
support  the  party  during  the  month's  stay  on  the  island. 

To  plan  and  carry  out  successfully  any  eclipse  expedition 
requires  not  only  scientific  ability  but  also  good  business  judg- 
ment. In  the  particular  case  we  are  considering  matters  were 
complicated  by  the  fact  that  once  on  the  island  we  were  shut 
off  from  any  help  from  the  outside  world  and  the  further  fact 
that  landing  on  Flint  Island  must  be  made  by  surf  boat  through 
a  narrow  passage  blasted  through  the  reef.  All  materials  that 
could  not  safely  be  floated  ashore  had  therefore  to  be  packed 
in  boxes  that  could  be  handled  easily  by  two  men  at  most. 

At  last  everything  was  ready,  and  the  party  of  six  from  the 
Lick  Observatory,  with  thirty-five  tons  of  freight,  sailed  from 
San  Francisco  for  Tahiti  (a  1 2-day s'  run)  on  November  22, 
1907.  At  Papeete  we  spent  three  busy  days  transhipping  our 
freight  to  the  Annapolis,  securing  a  supply  of  fresh  fruit  and 
other  perishables  and  a  surf  boat  for  landing,  and  picking  up 
our  Tahitian  carpenter,  cook  and  laborers.  On  the  evening  of 
December  7th  we  sailed  for  Flint  Island.  Two  days  later,  on 
the  forenoon  of  December  9th,  the  island  was  sighted,  a  small 
kite-shaped  patch  of  coral  rock  less  than  a  mile  wide  from  east 
to  west  and  slightly  over  two  miles  long  from  north  to  south ; 
22  feet  above  mean  sea  level  at  its  highest  point,  and  surrounded 
by  a  flat  coral  reef  beyond  which  could  be  seen  a  white  beach, 
strewn  with  shells  and  broken  coral,  sloping  gently  upward  to 
the  edge  of  a  grove  of  cocoanut  trees.  The  channel  blasted 
through  the  reef  on  the  northwest  side,  which  afforded  the 
only  possible  landing,  looked  narrow  indeed  and  to  our  unac- 


70  THE  ADOLFO  STAHL  LECTURES 

customed  eyes  the  surf  seemed  high.  Often,  we  had  been  told, 
it  was  impossible  to  land  at  all  and  ships  must  lie  off  shore 
sometimes  for  days  waiting  for  the  surf  to  subside.  To  our 
great  relief,  the  English  manager  of  the  cocoanut  plantation 
which  covers  the  island  told  us,  when  he  came  aboard,  that  the 
surf  was  really  low,  lower  in  fact  than  at  any  time  in  many 
months.  Landing  was  forthwith  rushed  and  by  eight  o'clock 
that  evening  all  of  our  effects  were  on  the  beach  and  the 
Annapolis  had  headed  again  for  Tahiti. 

Now  followed  days  of  strenuous  but  delightful  toil  amid 
surroundings  which  made  one  dream  of  Captain  Cook  and 
Robinson  Crusoe.  Simple  frames  for  huts  were  erected  from 
lumber  brought  with  us,  and  these  the  native  laborers  of  the 
plantation  covered,  both  roof  and  walls,  with  thatch  woven 
from  the  fronds  of  the  cocoanut  trees.  Canvas  or  colored 
calico  hung  from  cord  served  as  screens  for  the  doorways. 
Tents  were  run  up  to  shelter  the  instruments  and  the  supply- 
cases,  and  even  while  these  tasks  were  in  process,  the  alt-azi- 
muth instrument  was  mounted  upon  a  concrete  pier  and  the 
first  time  observation  secured  on  the  night  of  December  llth. 
A  meridian  line  was  also  run,  sites  chosen  in  the  cocoanut 
grove  where  a  few  trees  were  missing,  and  foundations  laid  for 
all  the  instruments. 

The  weather  was  delightful,  hot  indeed  in  the  Sun,  but 
tempered  by  the  constant  sea  breezes,  and  comfortable  enough 
in  the  shade  and  at  night.  Rain  fell  daily,  often  three  or  four 
showers  in  a  day.  It  was  a  novel  experience  to  be  interrupted 
in  the  course  of  time  observations  at  night  by  a  heavy  rainfall 
and  to  resume  observations  in  a  perfectly  clear  sky  within  half 
an  hour!  Or  to  be  at  work  by  day  at  the  adjustment  of  an 
instrument  and  to  hear  someone,  who  had  noted  a  low  cloud 
forming,  shout,  "Look  out  for  rain."  Then  to  seize  tarpaulin 
or  canvas,  hastily  cover  the  instrument,  and  be  drenched  by  a 
tropical  downpour  before  shelter,  a  few  yards  away,  could  be 
gained.  Half  an  hour,  sometimes  only  fifteen  minutes,  later, 
work  would  be  resumed  in  bright  sunlight  and  the  ground 
under  foot  would  be  practically  dry. 

Fully  a  week  before  the  day  of  the  eclipse  the  instruments 
were  all  practically  ready;  there  remained  only  those  last  fine 


PLATE  XV.     THE  INTRA-MERCURIAL  CAMERAS  AND  THE  MOVING- PLATE 
SPECTROGRAPH,  FLINT  ISLAND. 


A  TOTAL  ECLIPSE  OF  THE  SUN  71 

adjustments  which  make  the  difference  between  good  and 
excellent  results  and  the  rehearsal. 

The  duration  of  totality  was  3m  52s ;  to  utilize  those  precious 
seconds  to  the  utmost,  it  was  essential  that  every  one  should  be 
so  familiar  with  his  particular  duties  that  he  would  perform 
them  mechanically,  with  precision  and  the  maximum  rapidity. 
This  meant  rehearsal  and  dress  rehearsal,  so  to  say,  at  that. 
For  example ;  the  program  for  the  40- foot  telescope  called  for 
six  photographs  with  exposure  times  ranging  from  4s  to  64s, 
the  short  exposures  to  record  the  bright  inner  corona,  the  long 
one,  the  faint  outlying  streamers.  At  each  rehearsal,  and  there 
were  many,  the  observer  at  this  instrument  had  all  six  plate- 
holders  at  hand,  put  each  in  position  in  its  order,  went  through 
the  motions  of  exposing  the  plate,  stopping  the  exposure,  and 
putting  the  next  plate  in  position,  while  another  observer  called 
off  the  passing  seconds. 

The  morning  of  the  eclipse  dawned  bright  and  clear  and 
every  one  was  on  hand  early;  the  twenty  instruments  were 
given  their  final  inspection ;  the  plate-holders,  loaded  the  night 
before  with  the  plates  which  till  then  had  been  kept  in  sealed 
tin  cases,  were  put  at  hand ;  and  then  the  horizon  was  watched 
with  the  keenest  anxiety.  Would  rain  defeat  all  of  our 
preparations?  The  experience  of  the  preceding  days  led  us 
to  dread  it,  and,  in  fact,  at  eight  o'clock,  three  hours  before  the 
total  phase  began,  heavy  rain  did  fall.  Thereafter  the  sky  was 
alternately  clear  and  cloudy,  keeping  us  in  constant  suspense. 
Five  minutes  before  the  computed  time  of  second  contact  (the 
beginning  of  totality),  the  observer  at  the  chronometer  called 
off  the  time,  as  had  been  planned,  and  as  the  word  was  on  his 
tongue  a  dense  black  cloud  passed  over  us,  rain  began  to  fall, 
and  with  it  our  hearts.  Instruments  were  hastily  covered,  a 
native  perched  on  the  tower  of  the  40- foot  telescope  capped  the 
lens,  and  we  stood  by,  more  in  despair  than  in  hope,  as  the 
seconds  passed.  The  time-keeper  cried,  "two  minutes  before 
totality,"  in  a  rain  still  heavy  but  decreasing;  somewhat  more 
than  a  minute  later  the  slender  crescent  of  the  disappearing 
Sun  became  faintly  visible  through  thin  clouds.  These  grew 
rapidly  thinner,  and  two  observers  whose  special  duty  it  was, 
were  able  to  note  the  precise  second  when  the  total  phase  of 


72  THE  ADOLFO  STAHL  LECTURES 

the  eclipse  began.  Rain  .was  still  falling  in  scattered  drops,  but 
the  instruments  were  quickly  uncovered  and  the  program  was 
carried  out  as  planned.  Thin  clouds  covered  the  Sun  during 
nearly  half  the  total  phase,  but  during  the  last  half  only  a  very 
slight  haze  could  be  discerned.  Our  relief  and  joy  at  this 
almost  miraculous  good  fortune  are  more  easily  imagined  than 
described,  but  our  satisfaction  was  not  really  complete  until  it 
was  found,  upon  developing  them,  that  all  of  our  photographs 
were  excellent.  The  only  harm  the  rain  had  done  was  in 
preventing  our  taking  the  photographs  planned  for  the  first 
few  seconds  of  the  eclipse. 

The  cloud  which  so  nearly  ruined  the  plans  of  the  expedi- 
tion swept  over  the  island  at  a  very  low  altitude  and  was  quite 
limited  in  its  area,  hence  the  difference  of  only  a  few  feet  in  the 
location  of  an  instrument  meant  all  the  difference  between  a 
clear  sky  and  a  cloudy  one.  Thus,  Dr.  C.  G.  Abbot,  of  the 
Astrophysical  Observatory  of  the  Smithsonian  Institution,  who, 
with  an  assistant,  had  joined  our  party  at  San  Francisco,  had 
set  up  his  bolometric  apparatus  (designed  to  measure  the 
intensity  of  the  heat  radiation  from  the  corona)  on  the  beach 
near  our  landing  place,  perhaps  1000  feet  northwest  of  our 
station.  There  the  Sun  stood  in  perfectly  clear  sky  from  about 
15  seconds  before  the  total  phase  began!  An  English  party, 
headed  by  Mr.  Frank  McClean,  on  the  other  hand,  had  set  up 
its  instruments  about  200  feet  to  the  south  of  us.  There  the 
Sun  was  practically  wholly  obscured  during  the  first  half  of 
totality,  only  the  last  half  being  observable. 

It  took  us  nearly  a  month  to  set  up  and  adjust  our  instru- 
ments, but  it  required  less  than  two  days  to  take  them  down, 
pack  them,  and  get  them  aboard  the  Annapolis,  which  had  re- 
turned to  the  island  on  New  Year's  day.  The  photographic 
plates,  too,  had  all  been  developed,  dried,  and  packed  with  ex- 
treme care  in  sealed  tin  cases.  It  was  well  for  us  that  such 
speed  was  possible,  for  the  surf  was  rising  steadily  during  those 
two  days  and  at  eleven  o'clock  on  the  morning  of  January  5th, 
when  the  last  party  left  the  island,  the  expert  native  boatmen 
found  great  difficulty  in  sending  the  boat  through  the  surf. 
No  one  of  the  passengers  is  likely  to  forget  the  moments  when 
it  seemed  an  open  question  whether  they  would  succeed  or 


A  TOTAL  ECLIPSE  OF  THE  SUN 

whether  the  boat  would  be  flung  broadside  upon  the  outer  reef. 

Not  every  eclipse  party  can  expect  to  have  so  many  pleasant 
and  romantic  experiences  as  those  we  enjoyed  on  this  voyage 
to  the  South  Seas ;  but  wherever  the  station  may  be,  there  is  a 
fascination  about  even  the  most  prosaic  details  of  the  prepara- 
tion, and  a  joy  in  the  actual  observation  of  what  is  perhaps  the 
most  wonderful  and  beautiful  of  astronomical  phenomena  that 
afford  ample  compensation  for  all  the  time  and  trouble  and 
hard  work  an  eclipse  expedition  demands. 

The  American  Astronomical  Society  has  already  appointed 
a  special  eclipse  committee  to  make  general  plans  for  observing 
the  eclipse  of  June  8,  1918,  and  it  is  quite  certain  that  many 
American  observatories  will  send  out  parties  at  that  time.  The 
shadow  enters  the  United  States  on  the  coast  of  Washington  in 
latitude  +46°  50'  at  2h  55m,  Pacific  Standard  Time,  as  I  have 
already  said,  and  moving  rapidly  southeastward  leaves  the  land 
on  the  coast  of  Florida  shortly  before  sunset.  But  we  must 
remember  that  sunset  in  Florida  comes  when  the  Sun  in  our 
longitude  is  still  three  hours  or  more  above  the  horizon.  The 
Moon's  shadow  actually  sweeps  across  the  country  from  the 
Washington  coast  to  that  of  Florida  in  just  forty-seven  min- 
utes. A  number  of  cities  lie  close  to  the  central  line  of  the 
shadow7  path,  among  them  Baker  City  (Oregon).,  Hailey  and 
Montpelier  (Idaho),  Central  City  and  Denver  (Colorado), 
Jackson  (Mississippi),  and  Orlando  (Florida).  Denver  is  the 
site  of  the  Chamberlin  Observatory,  which  possesses  a  twenty- 
inch  refractor  adapted  for  photographic  as  well  as  visual  ob- 
servations. Professor  Howe  and  his  associates  can  observe 
the  eclipse,  therefore,  with  their  regular  observatory  instru- 
ments and  need  send  out  no  expedition.5  The  most  favorable 
locations  for  this  eclipse  are  unquestionably  on  the  line  from 
southeastern  Washington  through  Idaho  and  Colorado;  the 
Sun  during  totality  will  be  higher  in  the  sky  here  than  farther 
east,  the  eclipse  will  last  longer,  and  the  meteorological  condi- 
tions are  most  promising. 

The  eclipse  committee  has  made  no  report  as  yet,  but  it  is 
safe  to  forecast  the  general  nature  of  the  observations  that 
will  be  made,  and  their  purpose.  The  corona  will  certainly  be 

5  Unfortunately,  the  sky  was  cloudy  at  Denver  on  June  8,  1918. 


74  THE  ADOLFO  STAHL  LECTURES 

the  principal  object  of  study.  Large-scale  and  small-scale 
photographs  will  be  taken,  some  with  short  exposures  for  the 
brighter  portions,  some  with  long  exposures  for  the  fainter 
regions.  Intercomparison  of  these  photographs  will  enable  us 
to  build  up  a  true  picture  of  the  actual  corona.  Direct  photo- 
graphs, and  others  taken  with  the  spectroheliograph,  at  observ- 
atories like  the  one  on  Mount  Wilson,  on  the  day  of  the  eclipse 
and  on  the  preceding  and  following  days,  will  record  the 
number  and  location  of  the  sun-spots,  faculae  and  prominences, 
and  the  distribution  of  hydrogen,  calcium  and  other  gases  in  the 
upper  regions  of  the  Sun;  and  the  comparative  study  of  these 
photographs  with  those  taken  by  the  eclipse  expeditions,  will, 
it  is  to  be  hoped,  throw  new  light  upon  the  constitution  of  the 
corona  and  upon  its  relations  to  the  other  solar  envelopes. 

Spectrographic  observations  will  play  an  important  part  and 
spectrographs  of  several  different  types  will  be  used,  (1)  to 
record  the  general  coronal  spectrum  and  the  distribution  of 
coronal  light  in  the  spectrum;  (2)  to  determine  the  precise 
wave-lengths  of  the  coronal  lines,  especially  the  green  line  of 
coronium,  and  to  record  the  distribution  of  this  gas  at  least  in 
the  inner  corona;  (3)  to  photograph  the  violet  and  ultra-violet 
coronal  spectrum;  and  (4)  to  photograph  the  "flash  spectrum". 

Spectroheliographs  may  possibly  be  used  to  photograph  the 
chromosphere,  prominences  and  inner  corona ;  bolometers  will 
measure  the  intensity  of  the  radiation  of  the  corona  at  different 
distances  from  the  Moon's  limb,  and  special  magnetic  measures 
and  meteorological  observations  will  undoubtedly  be  made. 
Fairly  successful  "moving-picture"  records  have  been  secured 
at  one  or  two  recent  eclipses.  Several  such  records  ought  to 
be  made  at  different  stations  on  June  8,  1918.6 

Two  minutes  is  not  a  very  long  time,  but  a  single  expedition, 
well  planned  and  thoroughly  prepared  to  utilize  every  second 
to  the  utmost,  can  secure  most  valuable  material.  A  number  of 
parties  working  in  cooperation,  according  to  pre-arranged 
plans,  should  secure  data  that  will  mark  a  long  step  forward 
in  our  knowledge  of  the  Sun. 

As  to  observations  not  directly  relating  to  the  Sun,  it  is 
probable  that  search  for  an  intra-Mercurial  planet  will  not 


So   far   as   I   am   aware,   no   such   records   were   secured. 


A  TOTAL  ECLIPSE  OF  THE  SUN  75 

figure,  except  incidentally,  but  telescopes  of  the  same  type  as 
those  used  in  this  search  at  recent  eclipses — that  is,  batteries 
of  four  telescopes  of  about  11-foot  focal  length  so  mounted 
upon  a  single  polar  axis  as  to  give  simultaneous  photographs  of 
the  entire  region  about  the  Sun — will  undoubtedly  be  used  to 
test  the  relativity  theory  now  so  prominent  in  theoretical 
physics.  It  is  a  consequence  of  that  theory  that  a  beam  of  light 
passing  through  a  gravitational  field  should  be  deflected  from 
its  course  just  as  a  material  particle  traveling  with  the  velocity 
of  light  would  be.  If,  then,  a  star  is  so  situated  that  its  light  in 
falling  on  the  Earth  passes  close  to  the  limb  of  the  Sun,  it 
should  be  bent  in  toward  the  Sun  by  about  0.9".  If  it  passes 
20'  from  the  Sun,  the  deflection  is  less  than  half  as  great.  If 
two  stars  are  placed  on  opposite  sides  of  the  Sun,  their  light 
will  be  deflected  in  opposite  directions  and  the  effect  will  thus 
be  doubled.  Now  stars  so  nearly  in  line  with  the  Sun,  even  if 
bright,  can  be  photographed  only  at  the  time  of  eclipse.  Hence 
the  plan  is  to  take  plates  at  eclipse  time,  measure  the  distance 
between  star  images  suitably  placed  upon  them,  and  compare 
the  result  with  the  distance  between  the  same  stars  photo- 
graphed with  the  same  telescopes  at  an  earlier  or  later  season 
when  the  Sun  is  out  of  the  way.  If  the  attraction  of  the  Sun 
has  affected  the  direction  of  the  light  beam,  the  distance  on  the 
eclipse  plates  will  be  a  little  greater — the  amount  depending  on 
the  positions  of  the  stars — than  that  on  the  other  plates.  The 
theory  of  relativity,  while  far  too  technical  to  be  discussed  here, 
is  of  such  importance  to  our  fundamental  physical  concepts 
that  these  tests,  the  only  quantitative  observational  tests  that 
can  be  made  of  it  at  present,  will  be  of  the  greatest  interest. 


THE  MOON1 

By  ROBERT  G.  AUKEN 

One  Saturday  evening,  several  years  ago,  I  was  standing  in 
front  of  the  Lick  Observatory  with  a  party  of  people  who  had 
come  to  look  through  the  36-inch  telescope.  The  Sun  was  just 
setting  behind  the  hills  south  of  Mt.  Tamalpais,  and  as  it  dis- 
appeared, the  slender  crescent  of  the  Moon,  less  than  two  days 
past  the  new,  appeared  low  in  the  sky  south  of  the  sunset  point. 
One  of  the  visitors,  after  watching  it  a  moment,  turned  with 
the  question : — "Why  is  it  that  the  new  Moon  rises  in  the  west, 
while  the  full  Moon  rises  in  the  east?" 

As  soon  as  I  recovered,  I  explained  as  tactfully  as  I  could 
that  the  Moon  whether  full  or  new  always  rose  in  the  east,  but 
that  when  it  was  just  past  the  new-moon  stage  it  rose  very 
near  the  Sun  and  after  sunrise  and  therefore  could  not  be  seen 
until  the  Sun  had  set,  by  which  time,  of  course,  it  was  itself 
approaching  the  western  horizon.  But  my  tact  or  my  explana- 
tion, or  both,  were  unequal  to  the  occasion,  for  when  I  had 
finished,  the  visitor  replied  with  great  dignity,  "Well !  That  is 
the  way  it  may  do  here,  but  in  Humboldt  County  the  new  Moon 
always  rises  in  the  west !" 

That  any  one  should  be  so  ignorant  concerning  the  motions 
of  the  Moon,  is  certainly  hard  to  credit ;  but  my  visitor  differs 
only  in  degree  from  many  a  famous  poet  and  novelist.  I  could 
quote  a  description  of  a  sunset  in  a  story  written  by  one  of  the 
foremost  "realist"  fiction  writers  of  New  England,  and  pub- 
lished a  few  years  ago  in  Harper's  Monthly  Magazine,  in  which 
a  crescent  Moon  in  the  eastern  sky  adds  to  the  beauty  of  the 
scene ;  or  a  passage  from  a  novel  which  was  a  "best  seller"  not 
so  very  long  ago  and  whose  author  had  a  reputation  as  a 
scientific  man,  in  which  the  full  Moon  rises  at  midnight. 
Indeed  all  kinds  of  liberties  have  been  taken  with  the  Moon. 
Coleridge's  lines  in  The  Ancient  Mariner, 


1  Delivered    February    9,    1917. 


PLATE  XVI.    THE  MOON,  9o.  2.5n.  OLD. 


Photograph    taken   with    the   36-inch   refractor,   Oct.    11,    1891,    by   E.    S. 
Holden  and  W.   W.  Campbell. 


THE  MOON 

The  horned  Moon,  with  one  bright  star 
Within  the  nether  tip 

are  classic ;  and  we  have  all  in  our  childhood  recited  or  at  least 
read  The  Burial  of  Sir  John  Moore  with  the  line 

By  the  struggling  moonbeam's  misty  light. 

Some  critic  was  unkind  enough  to  look  up  the  almanac  and  he 
found  "that  the  Moon  was  new  on  the  16th  of  January,  1809,  at 
one  o'clock  in  the  morning  of  the  day  of  the  battle  of  Corunna." 
The  Moon  was  therefore  invisible  on  the  following  day,  and 
since  the  burial  took  place  on  the  night  after  the  battle,  it  was, 
in  any  event,  below  the  horizon. 

It  would  be  easy  to  cite  many  other  passages  in  which 
similar  errors  occur.  Nor  are  these  mistakes  confined  to 
writers  in  our  own  language.  William  Lyon  Phelps,  for 
instance,  in  his  Essays  on  Modern  Novelists,  says  that  "the 
Moon,  in  German  fiction,  is  not  astronomical,  but  decorative.  I 
have  read  some  stories  in  which  it  seems  to  rise  on  almost  every 
page  and  is  invariably  full.  Even  Herr  Sudermann  places  in 
Es  War  a  young  crescent  Moon  in  the  eastern  sky !" 

Our  modern  civilization  and  our  educational  system  are  to  a 
large  extent  responsible  for  this  general  ignorance  of  the 
apparent  motions  of  the  most  familiar  of  all  the  objects  in  the 
night  sky.  Astronomy,  in  our  country  at  least,  is  seldom  taught 
in  the  schools  and  generally  only  as  an  elective  in  our  colleges, 
and  boys  and  girls  can  pass  through  all  the  grades  to  a  uni- 
versity degree  without  acquiring  the  slightest  information  about 
the  Sun,  the  Moon,  the  planets  or  the  stars.  And  our  crowded 
hurrying  life  with  its  insistent  and  ever  growing  demands  upon 
our  time  affords  ever  less  leisure  for  quiet  observation  and 
thought,  and  city  lights  too  often  hide  from  us  the  lights  in  the 
sky.  That  is  why  I  am  devoting  the  first  part  of  this  lecture  to 
a  simple  account  of  the  Moon  as  we  see  it  in  the  sky. 

It  requires  no  observatory  equipment,  not  even  the  smallest 
telescope,  to  gain  a  knowledge  of  the  apparent  motions  of  the 
Moon  in  the  sky.  It  is  only  necessary  to  watch  it  with  seeing 
eyes,  as  the  ancients  did,  thousands  of  years  before  the  tele- 
scope was  invented.  Any  intelligent  boy  or  girl  can  repeat 
these  observations  and  verify  what  I  am  going  to  say,  and  I 
hope  that  many  of  you  who  hear  me  tonight  will  do  so.  When 


78  THE  ADOLFO  STAHL  LECTURES 

it  comes  to  the  real  motion  of  the  Moon  the  story  is  very 
different.  To  trace  this  motion  in  detail,  to  analyze  it,  and 
explain  it  on  the  Newtonian  theory  of  gravitation,  forms  one  of 
the  most  intricate  and  difficult  problems  of  mathematical 
astronomy.2  The  trouble  is  that  so  many  factors  enter.  If  the 
Moon  moved  simply  under  the  mutual  attraction  between  it  and 
the  Earth,  the  problem  would  be  the  comparatively  simple  one 
known  as  the  two-body  problem.  But  the  Sun's  attraction  is  a 
powerful  disturbing,  or,  in  technical  terms,  perturbing  force ; 
Venus  exercises  a  strong  attraction ;  the  other  planets,  in 
smaller  degree,  enter,  each  with  a  force  determined  by  its 
mass  and  distance ;  even  the  fact  that  the  Earth  is  not  a  sphere, 
but  bulges  at  the  equator,  is  a  factor  by  no  means  to  be 
neglected.  The  Moon,  therefore,  does  not  move  in  a  simple 
elliptic  orbit,  but  in  a  very  irregular  curve,  following  the  line  of 
the  ellipse  only  in  a  general  way,  and  it  is  so  near  the  Earth, 
relatively  speaking,  that  every  departure  from  simple  elliptic 
motion  is  detected  in  our  observations.  To  account  for  the 
observed  motion  under  the  law  of  gravitation,  taking  all  the 
disturbing  factors  into  consideration,  is  a  problem  that  has 
exercised  the  highest  powers  of  great  mathematicians  from 
Newton's  time  to  the  present  day.  We  may  well  be  proud  of 
the  fact  that  three  American  astronomers — the  late  Simon 
Newcomb,  the  late  George  William  Hill,  and  Professor  Ernest 
W.  Brown  of  Yale  University — have  taken  distinguished  parts 
in  the  solution  of  this  great  problem.  Professor  Brown's  lunar 
tables,  now  being  printed,  are  the  most  accurate  ever  con- 
structed. 

Returning,  after  this  digression,  to  the  Moon's  apparent 
motion,  the  diurnal  motion  due  to  the  rotation  of  the  Earth  on 
its  axis  is  the  first  to  be  noticed.  We  see  the  Moon  rise  above 
the  eastern  horizon,  circle  the  sky  towards  the  west,  and  set 
below  the  western  horizon.  The  points  of  rising  and  setting 
are  not  always  the  same  nor  does  the  Moon  cross  the  meridian 
always  at  the  same  altitude,  and  the  times  of  rising  change 
from  day  to  day.  The  observer  will  quickly  learn  to  associate 
the  times  of  rising  and  setting  with  the  Moon's  age  and  its 
phases.  For  a  day  or  two  at  new-moon  time  he  will  not  see  it 

-  See  Professor  Leuschner's  lecture,  on  a  later  page,  for  an  excellent  discus- 
sion of  the  motions  of  bodies  in  the  solar  system. 


THE  MOON  79 

rise  or  set  at  all.  Then,  if  he  is  sharp-eyed  and  the  air  is  very 
clear,  he  will  see  it  rise  shortly  after  sunrise,  a  slender  crescent. 
As  the  crescent  grows  from  day  to  day,  the  time  of  rising 
becomes  later  and  later  until,  when  the  crescent  has  rounded 
through  the  half-moon  and  gibbous  phases  to  full  moon, 
it  rises  in  the  east  about  the  time  the  Sun  is  setting  in  the  west. 
As  it  wanes  again,  first  to  the  half-moon  phase,  and  then,  in 
the  last  quarter  of  the  month,  to  an  ever  narrower  crescent, 
the  time  of  rising  grows  ever  later  until  we  see  it  rise  for  the 
last  time  in  the  month  just  before  sunrise. 


FIG.  7.    THE  PHASES  OF  THE  MOON. 

Of  course  this  retardation  in  the  time  of  its  rising  is  due  to 
the  fact  that  the  Moon  is  really  moving  about  the  Earth  from 
west  to  east.  Watch  it  for  a  few  hours  on  any  clear  moonlight 
evening  and  you  will  find  that  in  an  hour's  time  it  moves  east- 
ward among  the  stars  about  the  distance  represented  by  its 
own  apparent  diameter.  Continue  your  observations  and  in 
due  time  you  will  learn  that  it  requires  approximately  27^  days 
to  return  to  its  original  position  among  the  stars  so  far  as  its 
eastward  motion  is  concerned ;  but  now  it  may  be  a  little  farther 
north  or  a  little  farther  south  than  it  was  a  month  earlier.  This 
is  a  little  more  than  two  days  less  than  the  time  it  requires  to 
pass  from  new  moon  back  again  to  new  moon,  and  the  reason 
is  obvious  when  we  recall  the  fact  that  because  of  the  Earth's 


80  THE  ADOLFO  STAHL  LECTURES 

motion  in  its  orbit  the  Sun  also  seems  to  move  eastward  among 
the  stars.  In  a  month's  time  it  travels  over  nearly  %2  of  its 
orbit,  and  the  Moon  must  catch  up  with  it  before  it  can  again 
reach  the  new-moon  phase.  It  is  also  clear  that  the  phases 
must  in  some  way  be  related  to  the  change  in  the  Moon's 
position  with  respect  to  the  Sun,  for  full  moon  always  comes 
when  the  Sun  and  Moon  are  on  opposite  sides  of  the  Earth, 
new  moon  when  they  are  nearly  in  line  on  the  same  side. 

Note  the  Moon's  apparent  size  and  you  will  find  that  it  is 
always  about  the  same  but  that  it  does  vary  slightly.  At  one 
time  in  the  month  it  is  a  little  larger  than  the  average,  at 
another  a  little  smaller.  But  in  making  this  observation  be 
careful  to  watch  the  Moon  when  it  is  about  the  same  distance 
from  the  horizon,  for  it  always  looks  larger  when  near  the 
horizon  than  when  it  is  higher  in  the  sky.  This  is  an  illusion, 
for  the  Moon  is  really  nearly  4,000  miles  farther  away  when 
it  is  on  your  horizon  than  when  you  see  it  overhead — a  fact 
which  you  can  readily  demonstrate  by  a  simple  diagram — and 
actually  its  disk  is  then  a  little  smaller. 

After  careful  and  long-continued  observations  of  this  kind 
the  ancients  were  able  to  conclude,  in  the  first  place,  that  the 
Moon's  orbit  about  the  Earth — its  apparent  path  among  the 
stars — makes  an  angle  of  about  5°  with  the  ecliptic.  This 
explains  why  the  Moon  sometimes  rises  north  of  the  east  point, 
and  sometimes  south  of  it,  for  the  ecliptic  itself  makes  an  angle 
of  23^°  with  the  plane  of  the  Earth's  equator,  and  the  Sun 
is  south  of  the  equator  from  the  autumnal  equinox  (about 
September  21)  to  the  vernal  equinox  (about  March  21)  and 
then  north  of  it  through  the  next  six  months.  Let  us  note  just 
here  that  since  the  Moon  at  full  is  always  opposite  to  the  Sun, 
the  full  Moon  must  be  north  of  the  equator  during  our  winter 
months,  when  the  Sun  is  south  of  it,  and  south  of  the  equator 
during  our  summer  months.  The  full  Moon  therefore  "rides 
high"  in  the  sky  and  gives  us  the  most  light  in  the  winter  when 
we  have  the  least  sunlight,  and  rides  low  in  the  sky  in  the 
summer.  In  our  latitudes  this  is  not  a  matter  of  great  con- 
sequence, but  if  we  were  at  the  North  or  at  the  South  Pole,  it 
would  be  pleasant,  at  least,  to  have  the  Moon  above  the  horizon 
continuously  for  the  14  days  from  the  first  quarter  through  full 


PLATE  XVII.     THE  MOON,   19o.   12.5n.  OLD. 


Photograph   taken  with   the  36-inch   refractor,  Aug.   30,    1893,   £3;   A.   L. 
Colton  and  C.  D.  Perrine. 


THE  MOON  81 

Moon  to  the  last  quarter  every  month  during  the  long  polar 
night. 

Next,  the  ancients  learned  that  the  Moon's  distance  from  the 
Earth  varies  by  a  slight  amount,  corresponding  to  the  slight 
variation  in  its  apparent  diameter,  and  that  this  variation 
progresses  in  a  regular  manner,  completing  the  cycle  of  its 
changes  in  the  period  of  a  month.  This  we  now  know  is  due 
to  the  fact  that  its  orbit  is  not  an  exact  circle  but  is  flattened  a 
little  into  the  form  of  an  ellipse.  Of  course  they  also  learned 
that  the  Moon  does  not  shine  by  its  own  light  but  only  by 
reflected  sunlight.  This  led  to  an  understanding  of  the  phases 
of  the  Moon. 

A  careful  study  of  some  of  the  prominent  markings  on  the 
Moon's  surface  will  soon  convince  any  one  that  they  always 
remain  in  approximately  the  same  position  with  respect  to  the 
limb ;  that  is,  that  the  Moon  always  turns  the  same  face  toward 
the  Earth.  This  means  that  the  Moon  must  turn  once  on  its 
axis — make  one  complete  rotation — each  month.  That  is  a 
puzzling  statement  to  many  people  when  it  is  heard  for  the  first 
time  but  it  is  easy  to  show  that  it  is  true,  and  that  in  no  other 
way  could  the  Moon  keep  the  same  face  turned  toward  us.  Try 
walking  around  a  table  placed  near  the  center  of  a  room, 
always  facing  the  table  as  you  walk,  and  see  what  happens  t 
You  will  find  that,  in  making  the  round,  you  have  faced  each 
wall  of  the  room  in  succession ;  that  is,  you  have  yourself 
turned  once  completely  round  during  your  walk. 

I  said  just  now  that  the  Moon  always  keeps  the  same  face 
turned  toward  the  Earth.  This  is  true  in  a  general  way  but 
the  statement  is  not  quite  exact.  The  Moon's  equ'ator  is 
inclined  6^2°  to  the  plane  of  its  orbit,  consequently  at  one  time 
in  each  month  its  north  pole  is  tipped  61/*  °  toward  us,  and  two 
weeks  later  its  south  pole  is  similarly  tipped.  Therefore  we 
see  a  little  beyond  first  one  pole  and  then  the  other  each  month. 
This  slight  variation  we  call  the  libration  in  latitude.  Further, 
since  the  Moon's  orbit  is  an  ellipse  its  motion  in  its  orbit  will 
be  variable,  being  slower  when  it  is  farthest  from  the  Earth 
and  faster  when  it  is  nearest ;  but  its  motion  of  rotation  on  its 
axis  is  perfectly  uniform.  This  produces  what  we  call  the 
libration  in  longitude  and  permits  us  to  "see  alternately  a  few 


82  THE  ADOLFO  STAHL  LECTURES 

degrees  around  the  eastern  and  western  edge  of  the  lunar 
globe."  Finally,  the  Moon  when  it  rises  and  when  it  sets  is 
practically  on  a  plane  passing  through  the  center  of  the 
Earth  while  we  are  about  4,000  miles  above  that  plane ;  there- 
fore we  look  a  little  past  the  western  limb  of  .the  Moon  as  it 
rises  and  a  little  past  its  eastern  limb  as  it  sets.  The  net  result 
is  that  41/100  of  the  Moon  is  always  visible,  41/100  is  never 
visible,  and  the  remaining  18/100,  along  the  limbs,  is  some- 
times visible  and  sometimes  not. 

The  Moon  is  so  near  the  Earth  that  its  distance  can  be 
measured  with  very  great  accuracy.  One  method  of  doing  this 
is,  in  principle,  precisely  like  that  which  a  surveyor  employs  to 
determine  the  distance  to  an  inaccessible  object.  The  surveyor 
measures  off  a  base  line  of  suitable  length  from  both  ends  of 
which  the  object  is  visible.  At  each  end  he  then  measures  the 
angle  included  between  the  other  end  of  the  line  and  the  object. 
This  gives  him  a  triangle  in  which  he  knows  the  size  of  three 
independent  parts — one  side  and  two  angles — and  from  these 
he  can  readily  compute  the  other  parts.  In  the  case  of  the 
Moon  we  measure  its  distance  from  the  zenith  at  two  stations 
having  nearly  the  same  longitude  but  widely  separated  in 
latitude,  the  observatories  at  Greenwich,  England,  and  at  the 
Cape  of  Good  Hope,  South  Africa,  for  example.  Knowing  the 
latitudes  of  our  stations  we  have  for  our  base  line  the  length  of 
the  line  between  them  drawn  through  the  Earth's  crust,  and  the 
measures  of  the  Moon's  zenith  distance  supply  our  angles. 
Then  we  calculate  the  distance  from  each  observatory  to  the 
Moon  and  from  these  values  the  distance  to  the  Moon  from  the 
Earth's"  center.  The  mean  value  has  been  found  to  be  238,862 
miles ;  but  it  is  easier  to  remember  the  value  240,000  miles,  a 
round  number  that  is  sufficiently  exact  for  any  one  except  the 
specialist.  Having  the  Moon's  distance,  our  measures  of  its 
apparent  angular  diameter  can  be  converted  into  miles.  This 
leads  to  the  figures  2160  miles,  a  little  more  than  one-fourth 
the  diameter  of  the  Earth. 

Several  of  the  satellites  of  Jupiter  and  of  Saturn  are  fully 
as  large  as  or  even  larger  than  our  Moon,  but  the  planets  them- 
selves are  so  much  larger  than  the  Earth  that  the  contrast 
between  planet  and  satellite  is  very  much  greater.  Our  Moon, 


THE  MOON.  83 

in  fact,  ought  really  to  be  called  the  Earth's  companion  rather 
than  its  satellite.  Viewed  from  Venus  or  from  Mars  it  would 
easily  be  seen  without  the  telescope,  forming  with  the  Earth  a 
beautiful  double  star. 

It  is  its  nearness  to  us,  however,  rather  than  its  size,  that 
makes  the  Moon  the  only  body  except  the  Sun  which  exercises 
a  direct  influence  upon  our  lives  here  on  the  Earth.  I  am 
speaking  now  from  the  strictly  utilitarian  point  of  view. 
Planets  could  be  completely  destroyed  and  the  stars  hidden 
from  our  sight  and  in  one  sense  our  lives  would  go  on  without 
the  slightest  inconvenience,  though  our  intellectual  and  spiritual 
loss  would  be  immeasurable.  But  let  the  Moon  be  annihilated ! 
Immediately  the  effect  would  be  felt  in  nearly  every  shipping 
port  in  the  world.  The  ships  in  dock  could  not  get  out;  the 
ships  outside  could  not  get  in ;  and  the  maritime  commerce  of 
the  world  would  be  thrown  into  dire  confusion,  for  the  Moon 
is  the  principal  factor  in  producing  the  tides.  The  Sun  also 
raises  tides  on  the  Earth  but  its  effect  is  only  half  that  of  the 
Moon. 

We  cannot  enter  now  upon  the  story  of  the  tides ;  that 
would  make  a  lecture  in  itself.  But  I  want  to  take  up  one  point 
very  briefly.  If  the  Moon  raises  tides  upon  the  Earth,  then 
the  Earth  must  likewise  exercise  a  tidal  strain  upon  the  Moon 
and  because  the  Earth's  mass  is  so  much  the  greater  of  the 
two,  this  strain  must  be  about  20  times  that  exerted  by  the 
Moon  upon  the  Earth.  We  think  of  the  tides  as  a  phenomenon 
connected  with  the  ocean,  but  a  moment's  reflection  will  make 
it  clear  that  the  pull  of  the  Moon,  under  the  law  of  gravitation, 
is  just  as  strong  upon  the  solid  crust  of  the  continents.  The 
waters  of  the  ocean  are  freer  to  move,  that  is  all.  Now  it  can 
be  shown  mathematically  that  when  a  body  rotates  upon  its 
axis  in  the  same  direction  as  its  motion  in  its  orbit,  and  the 
rotation  time  is  shorter  than  the  revolution  period,  such  a  tidal 
force  acts  as  a  brake  to  slow  up  the  rotational  motion  until  the 
two  periods  are  equal.  It  is  thought  by  most  astronomers  that 
the  Moon  originally  rotated  much  faster  than  it  does  now  and 
that  the  cumulative  effect  of  the  Earth's  tidal  action  upon  it 
through  the  ages  is  responsible  for  the  fact  that  now  its  rotation 
time  equals  its  revolution  period,  in  other  words,  for  the 


84  THE  ADOLFO  STAHL  LECTURES 

observed  fact  that  it  now  keeps  the  same  face  always  turned 
toward  the  Earth. 

The  Moon  has  been  credited  with  many  other  influences 
upon  us,  malign  as  well  as  benevolent.  Our  words  lunacy  and 
lunatic  preserve  the  idea  once  universally  held  that  moonlight 
can  affect  the  minds  of  men ;  countless  wise  sayings  embalm  the 
belief  that  the  Moon  affects  the  weather ;  and  others,  the  belief 
that  the  planting  of  various  crops,  to  result  in  fruitful  harvests, 
must  be  timed  to  the  right  phase  of  the  Moon.  These  are  all 
superstitions,  worth  as  much  or  as  little  as  Tom  Sawyer's 
method  of  curing  warts.  Not  one  of  them  has  a  basis  of  fact, 
but  they  cling  tenaciously  to  men's  minds  and  still  influence  the 
actions  of  some.  In  a  certain  region  of  the  San  Joaquin  Valley, 
for  instance,  no  farmer,  even  today,  plants  his  cabbages  without 
first  consulting  an  almanac  to  see  whether  "the  Moon  is  right" ! 

Consider  the  Moon  and  the  weather.  We  are  told  that 
changes  in  the  Moon's  phases — at  the  quarters,  full  and  new— 
bring  changes  in  the  weather.  Now,  in  the  first  place,  the 
Moon  could  only  affect  the  weather  by  variations  in  the  amount 
of  heat  it  radiates  to  us.  There  is  a  variation  in  this  respect, 
it  is  true,  for  not  only  is  the  illuminated  surface  at  the  quarter 
phase  only  half  that  of  the  full  Moon  but,  because  of  the  rough 
surface  of  our  satellite,  this  surface  sends  far  less  than  half — 
only  one-ninth  or  one-tenth  as  much  light  and  heat  as  the  full 
Moon.  But  even  the  full  Moon  sends  so  little  that  it  can  have 
no  appreciable  effect ;  in  fact  it  sends  only  l/465,000th  as  much 
as  the  Sun.  Taking  the  phases  into  account,  it  is  found  that  in 
thirteen  seconds  we  receive  as  much  light  and  heat  from  the 
Sun  as  we  do  from  the  Moon  in  a  whole  year  !  Evidently,  then, 
the  Moon's  heat  is  quite  unimportant  to  us ;  a  light  cloud  pass- 
ing in  front  of  the  Sun  deprives  us  of  more  heat  than  the  Moon 
ever  sends  us.  In  the  second  place,  storm  centers  travel  across 
the  Earth,  generally  from  west  to  east  in  our  latitudes,  and  can 
often  be  traced  clear  across  the  continent,  or  even  half-way 
around  the  globe  in  the  course  of  a  week  or  two.  If  the  storm 
begins  with  a  change  in  the  Moon  at  one  station,  it  clearly  will 
not  begin  with  such  a  change  at  another  station  some  hundreds 
of  miles  east  or  west  of  the  first  one.  Finally,  records  kept  at 
many  stations  for  long  periods  of  time — a  hundred  years  in 


THE  MOON  85 

some  instances — show  no  relation  whatever  between  Moon 
change  and  weather  change,  though  chance  coincidences  are  of 
course  frequently  found. 

Now  let  us  look  at  the  Moon  itself  as  it  is  revealed  to  us  by 
the  telescope.  Our  first  surprise  is  to  find  the  surface  so 
extremely  broken  and  rugged ;  the  next  is  that  we  can  see  the 
details  of  all  the  features  so  clearly.  Visitors  to  the  Lick 
Observatory  often  ask  how  near  the  great  telescope  brings  the 
Moon  to  us.  This,  of  course,  depends  upon  the  magnifying 
power  we  use.  With  a  power  of  1000,  which  is  as  great  as  can 
be  used  to  advantage  under  ordinary  conditions  in  studying  the 
surface  of  a  planet  or  of  the  Moon,  it  is,  in  effect,  brought 
within  about  240  miles  of  the  Earth's  surface.  But  this  does 
not  give  quite  a  fair  idea  of  the  distinctness  with  which  we  see 
the  lunar  surface  details,  because  when  we  view  an  object  like 
a  mountain  240  miles  distant  on  the  Earth  we  are  looking  at  it 
through  a  much  denser  layer  of  our  atmosphere.  On  a  clear 
winter's  day  at  Mount  Hamilton,  for  example,  we  can  see  the 
Sierras  stretching  from  the  far  northeast  to  the  far  southeast 
and  can  readily  make  out  some  of  the  prominent  landmarks 
about  the  Yosemite  Valley,  180  miles  due  east  of  us,  without 
the  aid  of  glasses.  But  we  cannot  see  them  so  well  defined  as 
we  see  the  Moon's  features  through  our  telescopes.  Objects 
on  the  Moon  having  a  diameter  of  1,000  feet  are  easily  seen  and 
those  with  half  or  even  one- third  that  diameter  would  hardly 
escape  detection.  Small  inequalities  of  the  surface,  or  an 
ordinary  house,  a  single  tree  or  animal  or  plant  would  be 
invisible.  Rugged  as  the  Moon  looks  to  us,  its  actual  surface 
is  probably  rougher  still. 

On  that  side  of  the  Moon  which  is  visible  to  us,  there  are 
no  less  than  ten  mountain  ranges  of  considerable  extent, 
numerous  isolated  peaks,  some  10,000  cracks  or  "rills"  and 
more  than  30,000  "craters"  which  have  been  mapped  and,  for 
the  most  part,  named.  There  are  also  the  large  dark  areas 
which  from  Galileo's  time  have  been  known  as  mwria  or 
seas,  though  we  have  long  been  aware  that  they  are  dry.  The 
system  of  nomenclature  dates  back  to  Riccioli,  who,  in  1651, 
published  a  lunar  map  on  which  several  hundred  mountains 
and  craters  were  named  for  distinguished  astronomers  and 


86  THE  ADOLFO  STAHL  LECTURES 

mathematicians.  The  names  Alps  and  Apennines  and  a  few 
others  date  back  still  farther — to  1645,  when  Hevelius  con- 
structed the  first  satisfactory  map  of  the  Moon. 

It  is  not  my  purpose  to  describe  the  lunar  surface  in  detail, 
for  the  most  complete  and  vivid  description  I  could  possibly 
give  would  fail  to  convey  to  you  any  adequate  conception  of  its 
beauty  as  viewed  through  a  good  telescope.  What  little  I  shall 
say  will  be  said  in  the  hope  that  it  may  lead  many  of  you  to 
a  direct  study  of  our  companion  world.  Contrary  to  a  some- 
what general  impression,  a  very  large  and  expensive  telescope 
is  not  required.  A  good  lens  with  an  aperture  of  three  or  four 
inches,  driven  by  clockwork  if  possible,  or  mounted  upon  a 
simple  tripod,  and  supplied  with  eye-pieces  ranging  in  magni- 
fication from  50  to  150  or  200  diameters,  is  capable  of  yielding 
valuable  results  in  many  fields  of  astronomical  work  and  is 
certainly  amply  large  for  observations  of  the  Moon  undertaken 
primarily  to  gratify  one's  love  of  the  beautiful.3 

The  time  to  view  the  Moon  for  this  purpose  is  when  it  is 
crescent  in  the  first  quarter,  or  still  in  the  early  gibbous  phase 
in  the  second ;  or,  if  you  do  not  object  to  keeping  late  hours, 
when  it  is  again  waning  to  a  crescent  after  the  full-moon 
stage.  At  full,  the  Sun's  light  falls  upon  its  surface  so  nearly 
vertically  that  there  are  practically  no  shadows  and  hence  no 
contrasts.  Color  differences,  of  course,  exist  even  at  the  full- 
moon  phase,  and  are,  indeed,  conspicuous  to  the  unaided  eye. 
The  large  dark  areas  are,  in  general,  low  ground,  the  so-called 
maria,  or  seas ;  the  bright  portions  are  higher  ground.  The 


3  The  two  photographs  shown  in  Plates  XVI  and  XVII  have  been  selected  for 
reproduction  because  together  they  show  the  entire  visible  surface  of  the  Moon 
(neglecting  the  effects  of  libration)  at  phases  well  suited  for  general  telescopic 
views.  Many  observers  would,  however,  regard  the  views  at  phases  respectively 
a  day  or  two  earlier  and  a  day  or  two  later  as  still  finer.  The  large  crater  near 
the  center  of  the  terminator  edge  (sunrise  line)  at  the  right,  in  Plate  XVI,  is 
Copernicus;  to  the  left  of  it  and  below  stands  the  crater  Eratosthenes  at  the  head 
of  the  lunar  mountain  range  known  as  the  Apennines.  A  gap  in  the  range  nearly 
opposite  a  group  of  three  craters,  Archimedes  (the  largest),  Autolycus  and  Aris- 
tillus,  in  the  Mare  Imbrium  (at  the  right),  separates  the  Apennines  from  the 
Caucasus  Mountains  and  opens  into  the  Mare  Serenitatis  (at  the  left).  The  lunar 
Alps  extend  toward  the  right  from  the  lower  end  of  the  Caucasus  range  to  the 
great  crater  Plato  near  the  terminator.  Note  especially  the  narrow,  nearly  straight 
Valley  of  the  Alps  to  the  left  of  Plato. 

In  Plate  XVII,  Copernicus  is  the  great  crater  a  little  to  the  right  and  below 
the  center  of  the  illuminated  disk.  Note  the  complicated  system  of  ridges  and 
bright  streaks  radiating  from  it.  The  prominent  crater  near  the  top  of  the  photo- 
graph is  Tycho.  Bright  streaks  radiating  from  it  are  also  visible,  but  this  crater 
and  its  wonderful  system  of  bright  streaks  are  best  seen  at  the  full-moon  phase, 
when  they  form  the  most  conspicuous  and  indeed  almost  the  only  well-marked 
features  of  the  lunar  landscape. 


f        $»•      <: 


ARCHIMEDES. 
Aug.  15,  1888. 


ARCHIMEDES 
Aug.  27, 


PLATE  XVIII. 


Note  the  changes  in  aspect  produced  by  the  change  in  the  angle  of 
incident  light. 


THE  MOON  87 

floors  of  many  craters  are  also  dark,  whereas  the  remarkable 
systems  of  streaks  radiating  from  such  craters  as  Copernicus 
and  Tycho,  and  most  of  the  crater  peaks  and  walls,  are  bright, 
some  of  them  intensely  brilliant. 

But  it  is  only  when  the  sunlight  falls  slantingly  upon  them 
that  the  details  of  the  Moon's  surface  are  brought  out  in  strong 
relief.  Mountain  ranges,  isolated  peaks,  ringed  plains,  craters 
large  and  small,  and  canyons,  cracks  or  rills  are  now  distinctly 
recognizable.  It  will  be  noted  that  the  shadow  outlines  are 
extremely  sharp;  there  are  no  half-tones,  no  gradations  be- 
tween the  deep  blueblack  shadows  and  the  bright  sunlit  areas. 

As  the  phase  of  the  Moon  changes,  the  angle  of  incidence 
of  the  Sun's  light  grows  larger  or  smaller  and  the  shadows 
change  their  dimensions,  their  forms  and  their  positions.  The 
result  is  a  change  in  the  appearance  of  craters  and  peaks  that 
has  frequently  misled  observers  into  thinking  they  were  viewing 
true  physical  changes  on  the  Moon's  surface.  Observation 
continued  over  many  lunations  will  generally  dispel  this  idea. 
It  will  also  convince  you  that  there  is  at  no  time  any  evidence 
for  the  existence  of  clouds  above  the  surface  of  the  Moon  to 
obscure  the  view,  and  no  appearance  of  "weathering"  or  ero- 
sion on  any  of  the  rugged  mountain  slopes  or  crater  walls.4 

Now  the  sharpness  of  detail,  the  absence  of  clouds  and  of 
any  appearance  of  weathering  lead  to  the  inference  that  there 
is  no  water  and  no  air  upon  the  Moon.  This  inference  we  have 
every  reason  to  regard  as  correct ;  certainly  there  is  no  water  on 
the  Moon's  surface  and  if  any  atmosphere  at  all  is  present, 
which  is  very  doubtful,  it  must  be  extremely  tenuous — less  than 
a  thousandth  part  as  dense  as  that  of  the  Earth. 

It  is  a  fact  readily  verified  by  any  patient  and  careful 
observer  that  when  the  Moon  occults  a  star,  the  star  disappears 
instantaneously  at  the  Moon's  advancing  limb  and  emerges 
from  the  Moon's  receding  limb  with  equal  suddenness.  It  is 
also*  true  that  the  diameter  of  the  Moon  calculated  from  the 
duration  of  occultations  agrees  very  precisely  with  the  value 


4  Not  all  students  of  the  Moon  will  subscribe  fully  to  these  statements. 
Claims  have  been  made  from  time  to  time  by  trained  observers  entitled  to 
respectful  hearing  that  evidence  of  erosion  is  not  altogether  lacking  and  that 
(as  is  noted  on  page  93)  physical  changes  have  been  observed  in  certain  craters. 
Even  were  selenographers  not  divided  in  opinion  as  to  the  validity  of  these  claims, 
the  statements  made  above  would  hold  for  the  Moon's  surface  in  general. 


88  THE  ADOLFO  STAHL  LECTURES 

obtained  by  direct  measurement.5  But  even  a  very  tenuous 
atmosphere  would  bend  the  star's  rays  and  absorb  more  and 
more  of  their  light  the  nearer  the  rays  approached  to  the 
surface  or  limb  of  the  Moon.  Hence  the  star  would  disappear 
gradually  and  the  duration  of  an  occupation  would  be  con- 
siderably less  than  that  predicted  from  the  Moon's  measured 
diameter. 

This  is  perhaps  the  best  argument  to  prove  the  practical 
non-existence  of  a  lunar  atmosphere.  But  others  are  not 
lacking;  the  fact,  for  example,  that  at  an  eclipse  of  the  Sun 
the  Moon's  limb  is  perfectly  dark  and  sharp  upon  the  Sun's 
disk;  or  the  argument,  based  upon  what  is  known  as  the 
kinetic  theory  of  gases,  that  the  Moon's  mass  is  too  small  to 
enable  it  to  retain  an  atmosphere  even  were  it  to  be  endowed 
with  one.  But  we  need  not  carry  the  discussion  further,  for 
astronomers  are  agreed  upon  the  fact  that  the  Moon  is  essen- 
tially a  dead  world ;  a  world  without  air,  without  water, 
without  vegetation  and,  indeed,  without  soil,  unless  this  term 
be  given  to  volcanic  ashes  or  the  "cosmic  dust"  of  fallen 
meteors.  As  some  one  has  said,  the  Moon  is  a  world  without 
weather  and  where  nothing  ever  happens. 

The  most  distinctive  and  conspicuous  markings  upon  the 
Moon  are  the  almost  innumerable  craters.  They  are  found 
all  over-  the  visible  disk,  though  they  are  not  at  all  uniformly 
distributed ;  toward  the  south  pole  the  surface  is  fairly  honey- 
combed with  them,  whereas  the  broad  belt  of  the  chief  dark 
areas  or  maria  north  of  the  center  is  relatively  smooth.  In 
size  they  range  from  "craterlets"  barely  visible  in  the  most 
powerful  telescopes  to  the  great  ringed  plains  100  miles  or 
more  in  diameter.  One  writer  even  regards  the  lunar  Car- 
pathian, Apennine  and  Caucasus  mountains  as  but  the  frag- 
ments of  a  former  huge  crater  wall  which  had  a  diameter  of 
800  miles.  Generally  the  bounding  wall  is  approximately 
circular  and  is  compound,  "composed  of  shorter  ridges  wKich 
overlap  one  another,  but  all  trend  concentrically".  The  inner 


5  Direct  measurement  gives  slightly  the  larger  value,  but  this  is  due  chiefly  to 
irradiation.  Measures  of  a  bright  disk  like  that  of  the  Moon  or  of  one  of  the 
planets  are  always  a  little  in  excess  of  the  true  values.  An  excellent  illustration 
of  the  irradiation  effect  is  found  in  the  appearance  of  the  Moon  three  or  four 
days  after  new  Moon;  the  bright  crescent  appears  distinctly  larger  in  radius  than 
the  dark  portion  feebly  visible  by  reflected  earth-light,  a  fact  embodied  in  the 
phrase  "the  new  Moon  holding  the  old  Moon  in  its  arms". 


PLATE  XIX.     WALLED  PLAINS  ON  THE  (SUNSET)   TERMINATOR. 


The  upper,  double  walled  plain,  with  triple  central  peak  and  a  bright  rill 
connecting  the  peak  and  wall  at  the  right  is  Petavius.  Below  is 
Vendelinus.  Note  the  small  craters  on  the  walls  and  floor  of  Ven- 
delinus. 


THE  MOON  89 

plain  or  floor  is  lower  than  the  neighboring  outer  plain,  often 
thousands  of  feet  lower.  Theophilus,  for  example,  a  crater 
64  miles  in  diameter,  is  19,000  feet  deep.  The  crater  walls, 
as  a  rule,  slope  very  steeply  to  the  inner  floor  and  much  more 
gently  to  the  outer  plain.  Frequently  one  or  more  mountain 
peaks  tower  abruptly  from  the  inner  plain  of  a  large  crater  to  a 
height  of  even  11,000  feet  as  in  Copernicus,  or  16,000  feet  as  in 
Theophilus ;  but  these  peaks  never,  according  to  Neison,  reach 
the  altitude  of  the  crater  walls.  Finally,  the  craters  overlap 
one  another  in  almost  every  conceivable  way,  forming  com- 
plicated groups  and  chains,  and  smaller  craters  are  numerous 
on  the  floors  and  walls  of  larger  ones. 

Now  you  will  ask,  as  does  every  intelligent  visitor  to  the 
Lick  Observatory  after  seeing  the  Moon  through  the  telescope, 
"What  caused  the  craters?"  I  wish  I  could  tell  you!  But  if 
I  am  to  be  perfectly  honest  I  shall  be  obliged  to  confess  that  I 
do  not  really  know.  The  question  of  the  origin  of  the  various 
lunar  surface  features  is  one  on  which  astronomers  are  still  in 
doubt.  It  is  perhaps  not  difficult  to  conceive  of  the  formation 
of  the  mountain  ranges,  lofty  as  some  of  them  are,  and  of  the 
valleys  or  canyons  and  of  the  smaller  craters,  at  least,  by  forces 
similar  to  those  which  have  produced  corresponding  features 
upon  our  Earth,  especially  when  we  consider  the  fact  that, 
because  of  the  Moon's  smaller  mass,  a  given  force  acting 
against  gravity  there  would  be  about  six  times  as  effective  as 
here.  But  the  bright  lines  or  rays  running  out  from  some  of 
the  craters  are  unlike  anything  familiar  to  us  on  the  Earth's 
surface,  and  there  are  great  difficulties  in  the  way  of  accounting 
for  the  craters  themselves.  Of  the  many  theories  that  have 
been  proposed  at  one  time  or  another  we  need  here  examine 
only  the  two  which  at  the  present  time  command  the  serious 
attention  of  astronomers,  the  classic  "volcanic"  theory  and  the 
"meteoric"  theory. 

Probably  the  term  craters,  which  was  early  given  to  these 
formations  because  of  their  superficial  aspect,  has  been  the 
source  of  unconscious  prejudice  in  many  minds  in  favor  of 
the  volcanic  theory;  just  as  the  unfortunate  translation  of 
Schiaparelli's  Italian  term  canali  by  our  English  canals  has 
unquestionably  been  a  powerful  factor  in  creating  the  wide- 


90  THE  ADOLFO  STAHL  LECTURES 

spread  belief  in  the  artificial  origin  of  these  well-known  mark- 
ings on  Mars.  Be  that  as  it  may,  it  is  safe  to  say  that  a 
majority  of  astronomers  favor  the  volcanic  theory,  or,  to  use 
broader  terms,  the  theory  that  all  of  the  observed  configura- 
tions of  the  lunar  landscape  are  the  result  of  the  action  of 
forces  originating  in  or  on  the  Moon  itself.  Confining  our 
attention  to  the  craters,  using  this  term  generically  to  include 
both  large  and  small  formations,  we  find  that  this  theory 
encounters  a  number  of  difficulties. 

In  the  .first  place,  the  craters  are  so  numerous  and  many 
of  them  are  of  such  vast  dimensions  compared  with  the 
volcanic  craters  upon  the  Earth.  The  objection  as  to  disparity 
in  number  is  perhaps  fairly  met  by  the  assumption  that  the 
lunar  craters  were  formed  many  ages  ago  and  that  all  traces 
of  the  corresponding  early  volcanic  activity  on  the  Earth  have 
been  obliterated  by  later  processes  of  erosion  and  sedimenta- 
tion. It  is  not  so  easy  to  explain  the  relative  size  of  the  larger 
lunar  craters ;  and  the  facts  that  the  material  in  the  surround- 
ing walls  and  peaks  is  generally  not  sufficient  to  fill  the  crater 
bowls  and  that  there  is  little  or  no  evidence  of  lava  flows 
increase  the  difficulty.  It  is  certainly  hard  to  believe  that  the 
craters  on  the  Moon  were  formed  by  such  explosive  forces  as 
those  which  are  responsible  for  the  formation  of  Vesuvius,  and 
approximately  95  per  cent  of  all  known  craters  upon  the  Earth. 
Craters  of  the  subsidence  type,  like  those  on  the  Hawaiian 
Islands,  as  W.  H.  Pickering  and  others  have  shown,  bear  a 
much  closer  resemblance  to  the  lunar  formations ;  but  even 
here  the  resemblance  is  far  from  perfect  and  neither  type  of 
terrestrial  crater  has  any  features  similar  to  the  huge  central 
peaks  which  so  frequently  rise  from  the  floors  of  the  larger 
lunar  ones.  These  peaks  are  in  no  sense  secondary  crater- 
cones;  they  are  to  all  appearance  true  mountain  peaks. 

But  if  there  are  difficulties  in  the  way  of  fully  explaining 
the  lunar  markings  by  the  action  of  internal  forces,  the  objec- 
tions to  the  meteoric  theory  are  of  even  greater  weight.  The 
bombardment  must  obviously  have  been  a  terrific  one  by 
meteors  of  tremendous  size,  and  since  the  Earth  and  Moon 
revolve  about  the  Sun  together  in  the  same  general  path, 
craters  of  meteoric  origin  should  be  correspondingly  large 


THE  MOON 


91 


and  numerous  upon  the  Earth.  As  a  matter  of  fact,  however, 
the  largest  meteoric  mass  found  upon  the  Earth  could  have 
produced  but  a  puny  crater  compared  with  those  upon  the 
Moon;  and  the  only  crater  upon  the  Earth,  so  far  as  known, 
that  was  probably  formed  by  a  falling  meteorite  is  the  cele- 
brated "meteor-crater"  in  Arizona.6 


FIG.  8.  CRATER  MOUND,  ARIZONA. 
A  photograph  of  the  model  prepared 
for  Professor  C.  K.  Gilbert  is  shown 
in  a  (left)  ;  a  photograph  of  the 
topographic  map,  and  a  cross  sec- 
tion of  the  mound  are  shown  in 
b  (right). 


Again  the  objection  to  the  number  is  met  by  assuming  that 
the  lunar  craters  were  formed  in  the  early  history  of  the 
Earth-Moon  system,  and  it  is  also  argued  that  at  that  time 
meteors  of  far  greater  mass  than  any  known  in  historic  times 
may  have  been  encountered.  If  this  were  so,  it  would  seem 
that  there  should  be  more  than  a  little  indication  of  their 
former  existence  in  the  rock  strata  which  have  been  explored 
upon  the  Earth,  for  in  view  of  their  number  and  enormous 
masses  it  is  hardly  conceivable  that  all  traces  of  them  would 
disappear  after  they  had  buried  themselves  deep  in  the  ground, 
even  after  the  lapse  of  geologic  ages.  So  far  as  I  am  aware, 
however,  geologists  have  found  no  evidence  of  such  huge  falls. 
Indeed  it  must  be  said  that,  while  the  explorations  that  have 
been  made  of  the  Arizona  crater-mound  seem  reasonably  con- 
clusive as  to  the  method  of  its  formation,  the  meteoric  mass 
believed  to  be  responsible  has  not  been  discovered. 


6  This  is  known  also  as  Coon  Butte  and  is  situated  some  miles  east  of 
Canon  Diablo  near  Sunshine  Station  on  the  Atchison,  Topeka  and  Santa  Fe 
Railroad.  It  is  a  bowl-shaped  hole  approximately  three-quarters  of  a  mile  in 
diameter,  whose  walls  rise  about  150  feet  above  the  plain.  "The  bottom  of  the 
crater  is  about  570  feet  below  the  rim,  or  more  than  400  feet  below  the  general 
level  of  the  plain  outside."  For  an  interesting  description  of  this  crater,  see  the 
article  by  Elihu  Thomson,  from  which  I  have  just  quoted,  on  "The  Fall  of  a 
Meteorite,"  Proc.  Amer.  Acad.  Arts  and  Sci.,  47,  719,  1912. 


92  THE  ADOLFO  STAHL  LECTURES 

An  even  more  forcible  objection  to  the  meteoric  theory,  and 
one  that  to  my  way  of  thinking  is  insuperable,  arises  from  the 
predominantly  circular  form  of  the  lunar  craters,  large  and 
small.  Such  forms  imply  that  if  the  craters  were  produced  by 
meteors  these  must  have  fallen  vertically.  But  it  is  plain  that  a 
majority  of  meteors,  traveling  more  or  less  swiftly  through 
space  and  colliding  with  a  spherical  body  like  the  Moon,  must 
strike  the  surface  at  a  large  angle  to  the  vertical,  and  that 
numerous  encounters  must  be  mere  glancing  blows.  Hence 
we  should  expect  to  find  craters  and  scars  of  all  forms  rang- 
ing from  circular  pits  to  long  and  narrow  valleys,  the  oval 
pit  being  perhaps  predominant.  Observation,  however,  has 
revealed  only  two  lunar  markings  which  at  all  suggest  an 
origin  in  a  glancing  blow  from  a  meteor,  the  remarkable 
Valley  of  the  Alps,  and  the  valley  near  Rheita ;  and  forms 
intermediate  between  these  and  the  circular  craters  are  con- 
spicuously lacking.  This  objection  has  never  been  satis- 
factorily overcome. 

The  systems  of  bright  rays  or  streaks  about  Tycho, 
Copernicus  and  one  or  two  other  large  craters  are  puzzles, 
whatever  theory  of  crater  formation  we  adopt.  They  run  in 
nearly  straight  lines  over  craters,  cracks,  peaks  and  seas  alike, 
sometimes  for  hundreds  of  miles ;  and  at  no  phase  angle  do 
they  cast  shadows.  Hence  they  are  neither  elevations  above, 
nor  depressions  below  the  surrounding  surface.  Many  expla- 
nations of  their  nature  and  origin  have  been  offered  but  no  one 
of  these  is  at  all  satisfying. 

I  have  stated  the  objections  to  the  theories  rather  than  the 
arguments  in  their  favor  because  the  objections  must  in  some 
way  be  removed  before  either  theory  can  be  accepted  as 
satisfactory.  Some  recent  work  by  Professor  R.  W.  Wood, 
however,  may  be  referred  to  here  which  to  a  certain  extent 
seems  to  favor  the  theory,  of  origin  by  volcanic  or  other 
internal  forces.  He  has  photographed  the  Moon  in  light  of 
different  wave-lengths,  first  in  yellow  light,  then  in  violet  and 
finally  in  ultraviolet  light,  and  the  three  sets  of  photographs 
show  some  marked  differences  in  appearance.  For  example,  a 
large  dark  patch  just  above  the  crater  Aristarchus  appears  on 
the  ultraviolet  picture,  which  is  practically  invisible  in  the 


PLATE  XX.     COPERNICUS. 


Drawing  by  Professor  E.  L.  Weinek. 

From  the  negative  taken  at  the  Lick  Observatory  on  July  28,  1891,  15^  49m 

16»  P.  S.  T. 


THE  MOON  93 

yellow  one  and  only  faintly  visible  in  the  violet  one.  Professor 
Wood  took  two  specimens  of  volcanic  tufa  of  about  the  same 
color,  one  of  which  photographed  light  and  the  other  dark  in 
rays  of  ultraviolet  light.  Placing  a  small  chip  from  the  dark- 
specimen  upon  the  light  one  he  secured  effects  exactly  repro- 
ducing those  shown  by  the  Aristarchus  spot.  Analysis  then 
showed  that  the  dark  chip  contained  iron  and  traces  of  sulphur. 
Experimental  photographs  of  many  rock  specimens  having1 
iron  stains  failed  to  give  these  effects,  but  by  taking  the  speci- 
men of  tufa  which  had  photographed  light  in  the  ultraviolet 
picture  and  forming  on  a  spot  on  its  center  a  very  thin  deposit 
of  sulphur — so  thin  as  to  be  invisible  to  the  eye — he  obtained 
photographs  showing  the  spot  quite  black  in  the  ultraviolet, 
gray  in  the  violet  and  invisible  in  the  yellow.  This  makes 
it  appear  probable  that  there  is  a  deposit  of  sulphur  near 
Aristarchus  on  the  Moon.  More  extended  work  along  this 
line  is  needed,  however,  before  any  theory  of  crater  formation 
can  be  based  upon  it. 

I  have  said  that  the  Moon  is  a  world  where  nothing  ever 
happens.  Some  astronomers  would  take  exceptions  to  this, 
and  it  is  perhaps  well  to  remind  ourselves  that  a  universal 
affirmative  (or  negative)  is  a  dangerous  form  of  statement. 
It  is  quite  conceivable,  for  instance,  that  a  large  meteorite 
might  strike  the  Moon  at  some  time  and  that  we  might  be  able 
to  detect  the  effect.  Again,  the  surface  is  certainly  subjected 
to  extreme  variations  of  temperature;  there  is  no  atmosphere 
to  shield  it  from  the  direct  rays  of  the  Sun  during  the  two 
weeks  of  the  lunar  "day,"  or  to  blanket  it  during  the  two  ensu- 
ing weeks  of  the  lunar  "night".  Doubtless  some  cracking  of 
the  surface  must  from  time  to  time  result ;  but  it  is  question- 
able whether  this,  could  proceed  on  a  scale  large  enough  to 
become  visible  to  us. 

Physical  changes  have  repeatedly  been  reported  by  expert 
observers  in  connection  with  a  few  of  the  craters  and  in 
particular  with  the  relatively  small  crater  Linne  in  the  Sea  of 
Serenity  (Mare  Serenitatis)  ;  but  the  general  opinion  is  that 
the  reality  of  these  supposed  changes  has  not  yet  been  fully 
established  and  some  selenographers  assert,  on  the  other  hand, 


94  THE  ADOLFO  STAHL  LECTURES 

that  "no  eye  has  ever  seen  a  physical  change  in  the  plastic 
features  of  the  Moon's  surface". 

Very  positive  statements  are  also  made  by  certain  competent 
observers  that  slight  color  changes  take  place  in  the  course  of 
each  month  in  the  neighborhood  of  one  or  two  of  the  craters. 
Further  confirmatory  observations  are  desirable  before  we 
accept  these  changes  as  demonstrated;  and  even  then  we  may 
well  hesitate  to  accept  the  explanations  that  have  been  offered ; 
as,  for  example,  that  they  are  due  to  vapors,  issuing  from 
cracks  in  the  surface,  which  are  deposited  as  snow  or  hoar 
frost  in  the  lunar  night  and  evaporated  in  the  lunar  day; 
or  that  vegetation  of  a  low  order  springs  up,  runs  the  cycle 
of  its  life  history  in  each  lunar  day  and  perishes  in  the  cold  of 
lunar  night. 

My  conclusion  is  that  we  have  still  much  to  learn  of  the 
nature  and  origin  of  the  surface  markings  on  the  Moon, 
though  it  is  the  nearest  body  to  us  in  space.  It  may  be  a  dead 
world,  but  it  will  long  continue  to  be  an  interesting  object 
of  study. 


FIG.  1 — The  Diffuse  Nebulosity,  Messier  8,  in  Sagittarius. 


FIG.  2— The   Diffuse   Nebulosity, 
N.  G.  C.  II  5146. 


FIG.  3— The   Dumb-Bell    Nebula. 


PLATE  XXI. 


THE  NEBULAE1 

By  HEBER  D.  CURTIS 

In  the  four  lectures  of  the  Stahl  series  which  have  preceded 
this  one  you  have  heard  about  that  portion  of  the  universe  to 
which  our  own  little  Earth  belongs,  you  listened  to  what 
astronomy  has  to  say  regarding  the  planets  of  our  solar  system 
and  whether  they  may  possibly  be  inhabited  or  not,  learned 
something  of  those  mysterious  wanderers  in  our  system  which 
we  call  the  comets,  studied  the  surface  of  that  cold  and  lifeless 
satellite  of  ours,  the  Moon,  and  the  fact  was  brought  home  to 
you  that  the  mighty  Sun  was  only  our  own  particular  star, 
and  not  a  very  great  or  important  star  at  that  except  for  its 
position  as  the  center  of  our  solar  system.  In  the  present 
lecture  we  shall  consider  the  nebulae,  a  remarkable  class  of 
objects  in  the  universe  without,  a  universe  so  vast,  of  such 
incomprehensible  extent,  that  our  own  solar  system  is  but  an 
atom  in  comparison. 

Though  the  task  is  apparently  a  hopeless  one,  it  may  be  an 
advantage  if  we  make  the  attempt  at  the  start  to  realize  the 
vastness  of  this  outer  stellar  universe  of  which  our  solar  sys- 
tem forms  so  inconspicuous  a  part.  We  can  all  form  some  con- 
ception of  the  distance  around  the  Earth,  say  twenty-five 
thousand  miles,  and  we  can  then  have  some  sort  of  an  idea  of 
the  distance  of  the  Moon  as  about  ten  times  as  far  away.  But 
neither  the  layman  nor  the  professional  astronomer  can  form 
any  adequate  conception  of  the  distance  of  the  Sun,  ninety-three 
millions  of  miles  from  our  Earth.  Nor  can  we  have  any  idea  of 
the  distances  of  the  stars  from  the  fact  that,  at  the  distance  of 
the  average  naked-eye  star,  ninety-three  millions  of  miles  looks 
to  us  of  about  the  same  size  as  a  fifty-cent  piece  in  Los  Angeles, 
viewed  from  San  Francisco.  Out  in  this  ocean  of  space  a 
measuring  rod  a  million  miles  in  length  is  all  too  short ;  it  would 
be  like  trying  to  measure  the  distance  to  Los  Angeles  with  a 
foot  rule.  Something  a  million  times  larger  than  this  would 


1  Delivered    March    9,    1917. 


96  THE  ADOLFO  STAHL  LECTURES 

be  better,  so  the  foot  rule  which  the  astronomer  ordinarily 
uses  is  the  distance  traveled  by  light  in  one  year,  which  he  calls 
a  light-year.  A  light-year  is  nearly  six  trillion  miles ;  that  is, 
take  a  length  of  a  million  miles  and  lay  it  down  as  a  measure 
six  million  times,  end  to  end.  The  light-year  is  not  quite  six 
trillion  miles,  but  we  need  not  be  particular  about  a  few  billion 
miles,  more  or  less.  It  takes  light,  then,  over  four  years  at  the 
rate  of  186,500  miles  a  second,  to  reach  the  very  nearest  of  the 
stars,  so  such  a  star  is  said  to  be  four  light-years  away.  We 
feel  certain  that  some  of  the  celestial  objects  are  so  far  away 
that  it  takes  light  a  hundred  thousand  years  to  make  the 
journey,  in  other  words,  we  see  such  objects  not  as  they 
actually  are  tonight,  but  as  they  were  one  hundred  thousand 
years  ago.  But  enough  of  such  brain-staggering  figures.  It 
is  sufficient  if  we  from  these  facts  comprehend  a  little  more 
clearly  that  this  stellar  universe  is  something  wonderful  and 
mighty,  far  beyond  the  power  of  the  mind  of  man  to  grasp. 

What  we  term  the  factor  of  space-distribution  is  of  consid- 
erable importance  in  all  theories  of  the  nebulae,  that  is,  the 
way  in  which  these  are  arranged  with  reference  to  the  great 
mass  of  the  stars,  so,  at  the  start,  a  word  or  two  with  reference 
to  the  "'geography"  of  the  stellar  universe  will  be  in  place. 
We  can  see  for  ourselves  on  any  clear  night  that  most  of  the 
stars  appear  to  be  grouped  near  the  Milky  Way,  and  our  tele- 
scopes and  photographs  show  that  this  is  really  the  case ;  the 
stars  are  not  arranged  regularly  all  through  space,  but  the 
great  majority  of  the  thousand  million  or  so  of  stars  are 
grouped  in  a  relatively  flat  disk,  so  that  the  shape  of  the  stellar 
universe,  when  we  consider  the  stars  alone,  is  much  like  that  of 
a  thin  pocket  watch,  with  our  Sun  fairly  near  the  center. 

Another  point  which  will  be  of  importance  later  in  the 
lecture  is  what  we  may  term  the  factor  of  space-velocity.  All 
these  apparently  fixed  celestial  objects  are  really  moving  in  all 
directions  at  very  high  rates  of  speed.  Thus  we  may  not 
speak  of  the  part  of  space  occupied  by  our  solar  system,  but 
simply  of  the  part  of  space  which  it  now  occupies,  for  the  Sun 
and  all  his  retinue  of  planets  is  moving  through  space  at  the 
rate  of  about  twelve  and  a  half  miles  in  every  second  of  time. 
This  seems  inconceivably  rapid  to  us,  but  our  Sun  is,  even  at 


THE  MOON  97 

this  rate  of  speed,  quite  a  slow  coach  compared  with  many  of 
the  stars.  Thus,  when  the  Egyptians  commenced  to  study  the 
heavens  five  thousand  or  more  years  ago,  we  and  our  solar 
system  were  two  trillion  miles  from  where  we  are  tonight. 
At  that  rate  it  would  take  us  fifty  or  sixty  thousand  years  to 
reach  the  very  nearest  of  all  the  other  stars,  provided  we  were 
going  exactly  in  that  direction,  which  we  are  not.  When  you 
leave  the  hall  at  the  close  of  this  lecture  the  Sun  and  all  our 
system,  and  all  of  us  with  it,  will  have  traveled  about  forty-three 
thousand  miles  from  the  place  where  they  were  when  you 
sat  down.  If  I  should  happen  to  talk  ten  minutes  too  long  we 
should  be  seven  thousand  miles  beyond  the  corner  where  we 
should  have  got  off ! 

But  neither  the  two  trillion  miles  which  our  system  has 
traveled  since  the  days  of  the  Egyptians,  nor  the  equal  or 
greater  movements  which  all  the  stars  have  made  in  that 
interval,  have  made  any  essential  difference  in  the  general 
appearance  of  the  heavens,  for  two  trillion  miles  is  not  a  very 
long  way  as  distances  go  in  the  outer  world  of  space.  The 
Egyptian  saw  his  night  sky  tilted  at  a  different  angle  and  had 
a  different  pole  star  than  our  own,  because  of  a  progressive 
change  in  the  position  of  the  Earth's  axis,  but  the  constella- 
tions looked  practically  the  same  then  as  they  do  now ;  though 
all  the  stars  are  moving  at  these  rapid  rates  of  speed,  it  takes 
much  longer  than  five  thousand  years  for  these  motions  to 
show  so  as  to  be  very  perceptible  without  a  telescope  and 
accurate  measures. 

Now  out  in  this  limitless  ocean  of  space  we  see  just  two 
great  classes  of  objects,  the  stars,  and  the  nebulae;  while  our 
subject  is  the  nebulae,  the  stars  will,  of  necessity,  be  occasion- 
ally mentioned  as  well.  As  for  the  stars,  our  great  telescopes 
and  the  photographic  plate  tell  us  that  there  must  be  a  thousand 
million  or  so,  separated  from  each  other  and  from  us  by 
trillions  and  quadrillions  of  miles.  But  there  is  a  smaller 
number  of  objects  of  an  entirely  different  class  from  the  stars, 
objects  which  in  a  telescope  look  like  very  faint  luminous 
clouds,  which  is  the  reason  they  have  been  given  the  name 
"nebula"  from  the  Latin  word  for  cloud.  There  are  several 
hundred  thousand  of  these  nebulae,  ranging  in  apparent  size 


98  THE  ADOLFO  STAHL  LECTUEES 

from  mere  specks  to  great  masses  covering  a  sky  area  larger 
than  that  covered  by  the  Moon.  Only  a  very  few  of  them  are 
large  enough  or  bright  enough  to  be  seen  without  a  telescope, 
and  even  in  the  largest  telescopes  the  best  of  them  prove 
generally  to  be  very  disappointing  objects  to  the  layman,  as 
they  are  so  faint  and  indistinct.  Their  full  beauty  and  wonder- 
ful struct  lire  is  brought  out  only  by  photographs  of  several 
hours'  exposure  made  with  a  large  reflecting  telescope,  and  the 
illustrations  shown  were  made  in  this  way  by  the  Crossley 
reflector  at  the  Lick  Observatory. 

In  looking  at  reproductions  of  the  nebulae  it  is  well  to  try 
to  keep  in  mind  that  these  remarkable  objects  are  really  of 
enormous  size ;  perhaps  the  following  illustration  will  assist  in 
forming  this  impression.  We  shall  not  be  very  far  wrong  in 
the  statement  that  the  diameter  of  the  average  star  is  from 
half  a  million  to  a  million  or  more  miles.  Now  the  thing 
which  is  most  apt  to  disappoint  the  average  observatory  visitor 
as  he  looks  through  a  great  telescope  at  a  star  is  that  it  still 
looks  like  a  star,  a  mere  point.  He  sees  the  star  much  brighter 
than  it  would  appear  to  the  naked  eye,  but  expects  to  see  some- 
thing very  large,  filling  the  whole  field  of  the  telescope,  and  is 
at  some  difficulty  to  comprehend  why  the  brightest  star  should 
still  look  like  a  point  in  the  mightiest  telescope ;  it  is  hard  for 
him  to  realize  that  the  star  is  so  far  away  that  even  a  million 
miles  under  high  magnifying  power  looks  like  a  point  without 
size.  If  half  a  million  or  so  of  miles  has  no  size  at  all,  so  to 
speak,  at  stellar  distances,  how  mighty  must  a  nebula  be  which 
covers  a  space  equal  to  that  covered  by  the  full  Moon?  It 
win  then  be  evident  that  many  of  these  bodies  must  be  billions 
or  trillions  of  miles,  even  many  light-years,  across  from  edge 
to  edge. 

While  the  nebulae  take  a  great  variety  of  form,  there  are 
but  three  main  classes,  and  the  following  table  will  show  the 
TTmin  features  of  each  class. 

THE  GSEAT  DIFFUSE  NEBULAE 

Enormous  masses  of  luminous  matter ;  filmy,  cloud-like,  and  generally 
very  irregular.  Occur  in  or  near  the  Milky  Way  and  where  the  stars 
are  thickest.  Frequently  associated  with  "young"  stars,  never  with 
"old"  stars.  Speeds  low;  almost  at  rest  in  space.  Fairly  numerous. 


THE  NEBULAE  99 

THE  PLANETARY  NEBULAE 

Generally  small,  clear-cut,  bright,  and  with  a  central  star.  They  are 
gaseous  bodies.  Comparatively  rare  objects;  fewer  than  150  known. 
Tend  to  congregate  in  the  Milky  Way.  Average  speed  much  higher 
than  that  of  the  stars. 

THE  SPIRAL  NEBULAE 

Several  hundred  thousand  in  number;  generally  spiral  in  form. 
Congregate  about  the  poles  of  the  Milky  Way  where  stars  are  fewest, 
and  never  found  in  the  Milky  Way.  Speeds  enormous,  averaging  sev- 
eral hundred  miles  a  second.  Their  light  is  generally  the  same  as 
average  star-light. 

The  great  diffuse  nebulosities  are  wonderful  structures  and 
Fig.  1,  Plate  XXI,  shows  a  typical  object  of  this  class.  In 
some  cases,  as  in  the  nebulosity  around  the  Pleiades,  there  is 
good  reason  to  believe  that  the  light  from  these  nebulosities  is 
in  some  way,  perhaps  by  reflection,  caused  by  the  bright  stars 
with  which  they  are  associated.  But  in  the  majority,  as  the 
Great  Nebula  in  Orion,  Messier  8,  the  Trlfid  Nebula,  and 
others,  the  light  which  comes  to  us  from  them  tells  us  very 
clearly,  when  analyzed  in  our  spectroscopes,  that  these  are  truly 
gaseous  bodies.  They  contain  the  gases  hydrogen,  helium,  and 
something  which,  for  lack  of  better  knowledge,  we  call  "nebu- 
lium".  Just  how  they  shine  we  do  not  fully  know;  we  have 
evidently  to  do  here  with  matter  in  a  very  rare  and  perhaps 
primordial  state,  and  it  may  be  that  their  light  is  in  part  due 
to  some  form  of  electrical  excitation.  As  far  as  their  actual 
density  is  concerned  they  must  be  exceedingly  rare  bodies. 
Among  our  reasons  for  this  belief  is  the  easily  calculated  fact 
that  were  the  substance  of  the  enormous  nebula  in  Orion  any- 
where nearly  as  heavy  or  as  dense  as  ordinary  air  the  great 
mass  would  weigh  so  much  that  it  would  be  drawing  all  the 
neighboring  stars,  and  our  Sun  as  well,  swiftly  toward  it  by  its 
gravitational  power.  Sometimes  the  region  immediately 
around  one  of  these  diffuse  nebulosities  is  singularly  devoid  of 
stars.  Fig.  2,  Plate  XXI,  shows  this  in  a  striking  manner.  The 
best  explanation  appears  to  be  that  around  the  inner  luminous 
part  of  such  a  nebula  there  lies  a  great  mass  of  dark  matter 
which  obliterates  the  stars  in  the  background.  These  diffuse 
nebulosities  are  often  found  associated  with  stars  and  in  every 
such  case  the  star  is  one  of  the  class  which,  from  the  character 


100  THE  ADOLFO  STAHL  LECTURES 

of  the  light  it  sends  us,  is  believed  to  be  a  "young"  star ;  never 
do  we  find  this  diffuse  nebulosity  associated  with  stars  of  "old" 
types.  Bearing  in  mind  that  these  diffuse  nebulosities  are  al- 
ways found  in  or  near  the  Milky  Way  where  the  stars  are 
thickest,  we  can  see  that  there  are  very  good  reasons  for  sup- 
posing that  the  great  diffuse  nebulosities  may  well  be  regarded 
as  the  primordial  stuff  from  which  stars  are  made. 

Though  the  second  class  of  nebulae,  the  planetaries,  is  so 
small  a  one,  it  is  nevertheless  of  very  great  interest.  Fig.  3, 
Plate  XXI,  shows  a  typical  object  of  the  class.  Their  light 
shows  them  to  be  of  gaseous  constitution;  they  are  nearly  all 
rather  small  and  oval  or  round,  and  most  of  them  show  a  cen- 
tral star.  They  tend  to  congregate  in  the  Milky  Way  and  where 
the  stars  are  found  in  greatest  numbers.  Many  of  them  are  of 
exceedingly  complicated  structure,  and,  because  of  recent  dis- 
coveries with  the  spectrograph,  we  know  that  they  are  revolv- 
ing. But  they  are  a  very  puzzling  class.  We  do  not  know  as 
yet  how  they  can  take  these  complex  forms  and  show  certain 
motions,  as  they  do,  under  the  ordinary  laws  of  gravitation 
alone ;  perhaps  other  forces,  such  as  radiation  pressure,  come 
into  play  as  well.  We  would  like  to  think  of  them  as  in  that 
stage  of  nebular  condensation  and  stellar  evolution  which  comes 
just  before  true  stars  are  formed.2  But  there  are  several  diffi- 
culties in  the  way  of  accepting  this  theory.  In  the  first  place, 
the  planetaries  are  comparatively  rare  objects;  out  of  so  many 
hundred  thousand  stars  in  all  stages  of  development  it  is  very 
strange,  in  fact  inexplicable,  that  there  should  be  fewer  than  one 
hundred  and  fifty  at  this  particular  early  stage.  Then,  too,  their 
space  velocities  are  very  much  higher  than  that  of  the  average 
star.  Why  should  the  planetaries  stand  so  decidedly  apart  in 
this  respect,  and  how  can  this  gap  be  bridged  over  ?  Though  but 
a  theory  as  yet,  perhaps  the  most  acceptable  hypothesis,  because 
of  their  high  speeds  and  small  numbers,  is  that  the  planetary 
nebulae  are  to  be  regarded  as  a  somewhat  sporadic  case  in 
stellar  evolution,  arising  through  some  collision  or  cataclysm, 
and  not  to  be  regarded  as  cases  typical  of  the  general  run  of 
stellar  development. 

When  we  pass  on  to  the  third  subdivision,  the  great  class 

-  They  are  very  closely  allied  in  spectrum  with  a  comparatively  rare  class  of 
stars  known  as  the  Wolf-Rayet  stars. 


0    o 


GOO 


o  Oo 


FIG.  1 — At  left,  region  in  the  Milky  Way  showing  ten  to  twenty  thousand  stars, 
one  planetary  (N.  G.  C.  6563),  and  no  spirals. 

FIG.  2 — At  right,  region  near  N.  G.  C.  2507,  some  distance  from  the  Milky  Way, 
showing  few  stars  and  fifty-three  small  nebulae,  indicated  by  rings.  The 
area  of  each  half  is  somewhat  larger  than  that  covered  by  the  full  Moon. 


FIG.  3 — At  left,  is  a  drawing  of  the  Spiral  Nebula,  Messier  101,  made  by  Hunter 
in  1851  with  the  6-foot  reflector  of  Lord  Rosse. 

FIG.  A — At  right,  a  photograph  of  the  same  nebula. 


PLATE  XXII.     PHOTOGRAPHS  OF  SPIRAL  NEBULAE  BY  H.  D.  CURTIS;  DRAWING 
OF  MESSIER  101  BY  S.  HUNTER. 


THE  NcfiuLAEr-  -  •'  "^-     -Vj  -'^  101 

of  spiral  nebulae,  we  are  on  much  less  certain  ground.  Prior 
to  the  introduction  of  photography  there  were  fewer  than  ten 
thousand  nebulae  known.  It  was  Director  Keeler,  of  the  Lick 
Observatory,  who  first  really  showed  the  great  power  of  pho- 
tography and  the  reflecting  telescope  in  the  depiction  and  dis- 
covery of  nebulae.  Some  of  the  very  largest  of  this  last  class 
of  nebulae  had,  it  is  true,  been  seen  visually  to  be  of  spiral 
form,  but  Keeler's  photographs  showed,  first, — that  the  great 
majority  of  the  nebulae  were  spirals  in  form,  and,  secondly, — 
that  their  numbers  were  far  greater  than  had  before  been 
supposed.  Fig.  1,  Plate  XXII,  shows  a  small  part  of  the  Milky 
Way  where  the  stars  are  very  closely  packed  so  that  they  seem 
almost  to  touch  one  another,  though  in  reality  trillions  of  miles 
apart.  In  such  regions  as  this  we  never  find  a  single  spiral 
nebula.  Fig.  2  is  from  a  negative  taken  some  distance  from 
the  Milky  Way.  The  area  covered  is  somewhat  larger  than 
would  be  covered  by  the  disk  of  the  full  Moon,  and  it  will  be 
noticed  that  the  stars  are  comparatively  few  in  number.  But 
many  nebulae  are  seen  on  the  original  negative;  these  are  too 
faint  and  too  small  as  a  rule  to  show  in  the  cut,  so  the  position 
of  each  one  is  indicated  by  a  small  ring.  Most  of  these  small 
nebulae  are  probably  spirals.  It  may  be  seen,  then,  that  the 
spirals  occur  in  great  numbers  in  certain  definite  parts  of  the 
sky ;  the  estimates  as  to  their  total  number  range  from  two 
hundred  thousand  to  half  a  million.  A  recent  count  of  the 
small  spirals  occurring  on  all  available  regions  of  the  Crossley 
photographic  plates  taken  from  1898  to  1918  indicates  that  at 
least  700,000  small  spirals  are  within  reach  of  large  reflecting 
telescopes.  It  should  be  emphasized,  also,  that  they  never 
occur  in  the  regions  where  the  stars  are  thickest,  but  seem  to 
avoid  these  regions,  congregating  near  the  poles  of  the  Milky 
Way.  Figs.  3  and  4,  Plate  XXII,  will  serve  to  show  how  im- 
measurably photography  has  improved  our  knowledge  of  the 
nebulae.  The  first  is  a  copy  of  a  drawing  made  by  Mr.  Hunter 
in  1851  with  the  six-foot  reflector  of  Lord  Rosse,  and  the  other 
a  photograph  of  the  same  object.  It  will  be  evident  that  there  is 
simply  no  comparison  between  the  two,  and  that  the  beautiful 
and  delicate  structure  of  the  photograph  was  entirely  invisible 
in  a  powerful  telescope.  The  human  eye  is  a  wonderfully  deli- 


102  T*IK  ADOI.FO  STAIIL  LECTURES 

cate  instrument,  but  it  can  see  a  faint  object  no  better  or  more 
clearly  after  gazing  at  it  for  an  hour  than  it  could  in  the  first 
few  seconds ;  the  photographic  plate,  on  the  other  hand,  keeps 
adding  up  the  impressions  of  each  second  or  fraction  of  a  sec- 
ond it  is  exposed  to  the  object,  and  thus  with  long  exposures 
can  show  us  objects  far  too  faint  for  the  human  eye  alone, 
though  aided  by  the  greatest  telescope  in  existence. 

Though  the  general  characteristics  are  the  same,  the  spirals 
exhibit  a  great  variety  of  form.  Sometimes  there  are  but  two 
prominent  whorls,  as  in  Fig.  1,  Plate  XXIII ;  at  other  times  the 
structure  is  much  more  complicated,  as  in  Fig.  2.  Occasionally 
the  spiral  whorls  will  lie  so  close  together  that  a  ring  appear- 
ance is  shown,  but  in  most  cases  the  structure  is  more  open. 
Most  frequently  the  spiral  appears  like  an  elongated  oval  ( Fig. 
3),  because  it  is  essentially  a  flat,  disk-like  structure,  and  seen 
at  an  angle,  but  occasionally  it  lies  so  nearly  straight  across  our 
line  of  sight  that  it  appears  to  be  almost  round  (Fig.  4,  Plate 
XXII).  Then  again  we  see  quite  a  number  almost  exactly 
edge  on  and  can  get  a  vivid  idea  of  the  fact  that  the  spiral  is 
not  a  sphere  in  general  outline,  but  flat  and  lens-shaped.  Many 
of  these  edgewise  spirals  show  a  very  interesting  phenomenon. 
Figs.  4,  5,  and  6,  Plate  XXIII,  show  this  very  clearly.  There 
is  very  evidently  a  great  band  of  absorbing  matter  all  around 
the  circumference  of  these  spirals,  which  cuts  off  all  view  of 
the  matter  in  the  nebula  in  a  lane  running  along  its  length. 
Fig.  6  shows  this  in  a  most  striking  manner ;  the  dark  lane  is 
so  clear-cut  that  it  appears  almost  like  a  streak  of  black  paint 
along  the  image  of  the  nebula. 

Now  what  has  modern  astronomy  to  say  as  to  the  constitu- 
tion of  these  beautiful  objects,  the  spiral  nebulae?  May  we 
think  of  them  as  representing  a  certain  early  stage  in  the 
evolution  of  the  stars,  or  in  the  formation  of  such  a  system 
as  our  own  solar  system?  Do  they,  as  was  long  held  by 
astronomers,  give  us  ocular  evidence  in  support  of  some  sort 
of  nebular  hypothesis,  and  are  they  the  existing  representatives 
of  that  primeval  stage  when  our  own  solar  system  was  an 
extended,  whirling  mass  of  primordial  gas?  Is  it  possible  to 
regard  them  as  in  the  first  of  the  stages  so  well  put  by 
Tennyson  in  "The  Princess"  ? — 


FIG.  1— The  Spiral  Nebula,  N.  G.  C.  7479.      FIG.  2— The  Spiral  Nebula,  Messier  51. 


'IG.  3— The  Spiral  Nebula,  N.  G.  C.  253.  FIG.  4— N.  G.  C.    891. 

FIG.  5— N.  G.  C.  7814. 

FIG.  6— N.  G.  C.  4594. 

PLATE  XXIII. 


THE  NEBULAE  103 

This  world  was  once  a  fluid  haze  of  light, 
Till  toward  the  center  set  the  starry  tides 
And  eddied  into  suns  that,  whirling,  cast  the  planets. 

From  the  form  of  the  spiral  nebulae  we  feel  certain  that 
they  must  be  in  rotation ;  we  have  some  slight  evidence  of  this 
in  the  fifteen  or  twenty  years  during  which  they  have  been 
under  photographic  observation,  and  the  temptation  is  a  strong 
one  to  place  these  great  rotating  spirals  as  a  first  stage  in  the 
evolution  of  stars  or  solar  systems.  The  majority  of  astrono- 
mers still  believe  that  our  solar  system  was  formed  in  accord- 
ance with  some  sort  of  nebular  hypothesis,  though,  for  a 
number  of  weighty  technical  reasons  impossible  to  detail  here, 
the  well-known  nebular  hypothesis  of  Laplace,  in  just  the  form 
in  which  he  put  it  forward,  is  no  longer  accepted. 

But,  tempting  as  such  a  theory  of  the  spirals  is,  there  are  a 
number  of  very  strong  objections  to  it,  objections  which  depend 
largely  upon  the  two  factors  of  space  velocity  and  space  dis- 
tribution, which  were  mentioned  briefly  at  the  beginning  of  the 
lecture.  We  may  sum  up  in  the  following  table  what  we  know 
at  present  of  the  space  velocities  of  the  various  classes  of 
objects  in  the  stellar  universe.3 

THE  FACTOR  OF  SPACE  VELOCITY 

Diffuse  Nebulosities ;  velocities  low. 

The  Stars;  velocities  appear  to  increase  with  stellar  age. 

Class  B ;   average   speed     8  miles   per   second 

Class  A          "  "        14 

Class  F  "  '«        18 

Class  G          "  "        19 

Class  K          "  "        21        "       '" 

Class  M          "  "        21 

The  Planetary  Nebulae;  average  speed  48  miles  per  second. 
The  Spiral  Nebulae ;  average  speed  480  miles  per  second.4 


"  The  stars  are  divided  into  a  relatively  small  number  of  types  or  classes  in 
accordance  with  the  character  of  the  light  they  send  to  us.  Classes  B  and  A  are 
the  bluer  stars,  in  whose  light  hydrogen,  helium  and  other  gases  are  prominent,, 
and  are  generally  supposed  to  be  the  youngest  stars;  Classes  F  and  G  are  yellower, 
more  like  our  Sun,  and  show  the  presence  of  many  metals;  Classes  K  and  M  are 
redder  and  thought  to  be  stars  of  relatively  advanced  age.  While  other  arrange- 
ments of  these  classes,  as  indicating  relative  star  ages,  have  been  put  forward,  the 
generally  accepted  order  of  stellar  age  is  as  given  in  the  table. 

4  The  speed  given  for  the  spiral  nebulae  is  somewhat  uncertain,  as  this  has 
been  observed  for  a  comparatively  small  number  of  spirals  as  yet.  The  assump- 
tion is  also  made  that  their  motions  are  in  all  directions.  Future  work  may 
change  the  value,  but  it  seems  certain  that  it  will  remain  very  large. 


104  THE  ADOLFO  STAHL  LECTURES 

It  will  be  seen  that,  as  far  as  their  space  velocities  are  con- 
cerned, the  great  diffuse  nebulosities  fit  in  well  as  a  starting 
point  in  the  evolution  of  the  stars,  and  we  have  seen  that  these 
are,  if  associated  with  stars,  always  connected  with  those 
classes  of  stars  which  are  believed  to  be  the  youngest.  On  the 
other  hand,  the  planetaries  do  not  fit  in,  unless  we  should  place 
them  at  the  end  of  the  stellar  progression,  or,  as  is  perhaps 
better,  regard  them  as  exceptional  cases.  And  the  spiral 
nebulae  do  not  fit  in  at  all ;  their  almost  unbelievable  velocities 
place  them  in  a  class  entirely  apart  from  the  great  mass 
of  the  stars. 

Taking  up  the  even  more  important  factor  of  space  distribu- 
tion, the  following  table  will  show  roughly  the  apparent  loca- 
tion of  the  spiral  nebulae  with  reference  to  the  universe  of 
stars  which  we  call  our  galaxy. 

THE  FACTOR  OF  SPACE  DISTRIBUTION 

100,000  ±   Spiral  Nebulae 
Distance  unknown 


The   Milky  Way  and   stellar  universe 

is  believed  to  be  roughly  lens-shaped  and  about 

3,000  by  30,000  or  more  light-years  in  extent.    In  this  space 

occur  nearly  all  the  stars,  nearly  all  the  diffuse  nebulosities,  nearly  all 

the  planetary  nebulae,  nearly  all  new  stars,5  nearly  all 

clusters,  nearly  all  the  variable  stars,  etc.,  but 

NO  SPIRAL  NEBULAE. 


100,000  ±   Spiral  Nebulae 
Distance  unknown 

The  factor  of  space  distribution  is  then  entirely  at  variance 
with  the  hypothesis  of  the  spiral  nebula  as  a  starting  point  in 
the  formation  of  stars  or  of  our  own  solar  system. 

The  spirals  are  intrinsically  so  very  faint  that  it  is  a  matter 
of  great  difficulty  to  secure  spectroscopic  observations  which 


5  Except  the  new  stars  recently  found  in  spiral   nebulae  which  are   referred  to 
later. 


THE  NEBULAE  105 

will  throw  additional  light  on  their  composition,  and  this  work 
has  been  done  for  only  a  few  of  the  brightest  members  of  the 
class.  Here  we  find  a  very  puzzling  fact.  The  light  which 
these  objects  send  to  us,  when  analyzed  in  our  spectroscopes, 
tells  us  that  they  are,  in  general,  not  gaseous,  but  of  such  con- 
stitution that  their  light  is  just  the  same  as  would  be  expected 
to  come  from  a  great  cloud  of  stars.  The  future  may  possibly 
bring  to  light  new  facts  which  will  enable  us  to  give  some 
other  explanation,  but  our  present  evidence,  so  far  as  it  goes, 
leads  to  the  belief  that  the  spirals  are  composed  of  great  clouds 
of  stars  so  infinitely  distant  that  we  can  not  make  out  the 
individual  stars,  much  as  our  own  Milky  Way,  which  is  seen 
in  the  telescope  to  be  made  up  of  millions  of  closely  packed 
stars,  to  the  unaided  eye  appears  as  a  faint,  nebulous,  luminous 
band  across  the  sky. 

This  characteristic  of  their  light,  then,  together  with  their 
peculiar  distribution,  as  a  class,  apparently  apart  from  our 
stellar  system,  has  given  rise  to  what  is  known  as  the  "island 
universe"  theory  of  the  spiral  nebulae,  namely,  that  these 
objects  are  really  separate  galaxies  or  universes  of  stars.  There 
are  some  difficulties  in  the  theory,  but  we  have  at  present  very 
little  evidence  to  make  any  other  theory  of  the  spiral  nebulae 
a  more  probable  one.  On  this  theory,  too,  could  we  be  trans- 
ported out  into  space  a  distance  of  hundreds  of  thousands  or 
millions  of  light-years,  to  where  the  spirals  are  situated,  and 
look  back  from  that  point  at  our  own  particular  Milky  Way 
and  stellar  universe,  it  would  perhaps  appear  to  us  as  a  spiral 
nebula.  The  attempt  has  been  made  to  depict  our  stellar 
universe  as  a  spiral  in  general  arrangement,  with  the  Sun 
located  fairly  near  to  the  center  of  the  spiral. 

Why,  it  may  be  asked,  should  our  galaxy  be  situated  thus 
in  such  a  peculiar  way,  about  half-way  between  two  great 
groups  of  other  universes  at  enormous  distances  from  us,  with 
no  other  universe  relatively  close  to  our  own,  and  with  none  at 
all  visible  around  the  edge  of  our  own,  that  is,  beyond,  the 
circumference  of  our  Milky  Way?  This  peculiar  arrangement 
is  indeed  a  puzzle  on  any  theory  of  the  spirals.  Perhaps  the 
only  explanation  which  can  be  suggested  is  that  outside  our 
Milky  Way  and  nearly  in  its  plane  is  a  great  ring  of  absorbing 


106  THE  ADOLFO  STAHL  LECTURES 

matter  somewhat  like  those  which  are  found  in  certain  edgewise 
spirals  (Figs.  4,  5,  and  6,  Plate  XXIII),  and  that  this  matter 
cuts  off  from  our  view  all  other  universes  which  we  would  ex- 
pect to  lie  beyond^  and  in  line  with  our  Milky  Way.  Then,  too, 
the  analogy  of  our  Milky  Way  as  a  spiral  must  not  be  expected 
to  prove  too  much.  Nearly  all  spirals  have  a  well-defined  con- 
centration at  the  center,  marked  either  by  an  almost  stellar 
nucleus  or  by  an  almost  spherical  enlargement  of  the  nebular 
mass  around  the  center,  as  is  well  seen  in  numerous  photo- 
graphs of  edgewise  spirals.  There  is  pretty  certainly  no  such 
great  concentration  of  stars  at  the  center  of  our  own  galaxy, 
supposing  that  it  is  to  be  regarded  as  a  spiral.  However,  there 
are  a  number  of  very  flat  spirals  which  show  no  such  mass 
concentration  at  the  center,  but  they  are  scarcely  typical  of 
the  class. 

As  a  substitute  for  the  Kant-Laplace  nebular  hypothesis 
Professors  Chamber lin  and  Moulton  have  recently  propounded 
what  has  been  called  the  planetesimal  hypothesis  as  an  explana- 
tion of  the  spiral  nebulae  and  the  evolution  of  our  solar  system. 
The  theory  postulates,  in  brief,  that  a  spiral  nebula  would  be 
formed  as  a  result  of  the  disruptive  tidal  effects  produced  by 
the  close  approach  of  two  massive  stars.  It  has  been  well 
worked  out  and  appears  very  plausible  in  what  may  be  termed 
its  mathematical  and  mechanical  aspects ;  it  does  not  seem 
impossible  that  our  solar  system  might  thus  have  been  formed 
from  a  diminutive  spiral  nebula.  But  the  theory  can  as  yet 
offer  no  explanation  for  the  fact  that  the  light  sent  us  by  the 
spirals  seems  to  be  the  same  as  that  from  a  cloud  of  stars,  nor 
for  their  phenomenal  space  velocities.  Why,  too,  if  the  spirals 
are  formed  from  close  stellar  approaches,  should  we  find  them 
where  the  stars  are  fewest,  and  never  occurring  where  the 
stars  are  thickest  and  where,  if  at  all,  close  stellar  approaches 
should  be  common  ? 

It  must  be  admitted  that  the  evidence  at  present  available, 
upofi  which  any  satisfactory  theory  of  the  spiral  nebulae  may 
be  based,  is  exceedingly  scanty,  and  the  confession  must  be 
made  that  in  this  class  of  objects  modern  astronomy  finds  one 
of  its  most  perplexing  problems. 

A  promising  line  of  evidence  has  been  recently  developed 


North 


FIG.  1— The  Spiral  Nebula  N.  G.  C.  4527. 

Left:     May  8,  1901. 

Right:     April  16,  1915,  showing  Nova. 


North 


FIG.  2— The  Spiral  Nebula  N.  G.  C.  4321. 

Left:      April    19,    1901,    showing    Nova    A. 
Right:     March  2,    1914,   showing   Nova   B. 

PLATE  XXIV.     NOVAE  IN   SPIRAL  NEBULAE. 
Crossley  Reflector  Photographs. 


THE  NEBULAE  107 

by  the  discovery  of  novae  in  the  spiral  .nebulae.  More  than  a 
dozen  such  new  stars  have  been  found  in  spirals,  nearly  all 
within  the  last  year  or  two ;  two  of  these,  the  nova  in  the 
Andromeda  Nebula,  and  Z  Centauri,  were  moderately  bright; 
the  others  have  been  from  the  14th  to  the  19th  magnitude  at 
maximum.  So  far  as  can  be  decided  for  such  faint  objects, 
these  novae  are  apparently  similar  in  all  respects  to  the  new 
stars  which  have  appeared  in  our  own  galaxy  in  historic  times 
to  the  number  of  26.6  The  line  of  argument  based  on  the 
occurrence  of  these  novae  in  spirals  is  one  which  is  easily 
followed,  though  a  striking  analogy  is  by  no  means  a  rigid 
proof.  If  the  spirals  are  in  truth  island  universes,  composed 
in  each  case  of  a  thousand  million  or  more  stars,  we  should 
expect  to  observe  in  them  occasional  new  stars,  such  as  are 
observed  in  our  own  galaxy.  Moreover,  the  average  magnitude 
of  the  new  stars  in  our  own  system  has  been  about  the  fifth, 
while  those  seen  in  the  spirals  will  average  about  the  fifteenth 
magnitude  or  fainter;  that  is,  approximately,  ten  thousand 
times  less  brilliant.  If  we  assume  that  we  have  sufficient  of 
each  type  of  nova  to  afford  a  fair  average,  and  assume  in  addi- 
tion that  such  novae,  whether  in  our  own  system  or  in  the 
spirals,  are  bodies  of  the  same  order  of  size,  and  that  questions 
of  absorbing  matter  in  space  may  be  neglected  in  the  problem, 
we  may  then  postulate  the  spirals  as  V  10,000  or  one  hun- 
dred times  as  far  away,  on  the  average,  as  the  novae  which 
have  appeared  in  our  galaxy.  Now  all  these  latter  are  Milky 
Way  objects  and  probably  at  an  average  distance  from  us  of 
at  least  ten  thousand  light-years.  On  this  line  of  argument 
the  spirals  would  be  distant  from  us  one  million  light-years 
(more  probably  ten  or  one  hundred  times  this  distance,  as  the 
fainter  novae  in  spirals  would  escape  detection).  Our  own 
galaxy,  if  we  assume  its  diameter  as  thirty  thousand  light- 
years,  would  appear  only  10'  in  diameter  if  viewed  from  a 
distance  of  ten  million  light-years. 

The  peculiar  grouping  of  the  spirals,  in  that  they  are 
apparently  so  definitely  arranged  with  reference  to  the  plane 

6  It  must  be  noted,  however,  that  eleven  novae  have  been  found  in  the  great 
Andromeda  Nebula  alone,  and  nearly  all  of  these  within  the  last  two  years  (1916- 
1918).  If  they  are  intrinsically  as  bright  as  the  novae  which  have  appeared  in 
our  own  galaxy,  and  are  similar  to  them  in  their  origin  and  in  other  respects,  this 
is  a  remarkably  large  number. 


108  THE  ADOLFO  STAHL  LECTURES 

of  our  own  Milky  Way,  has  convinced  some  astronomers  that 
they  must  necessarily  be  connected  with  our  own  galaxy,  on 
the  ground  that  so  definite  a  relationship,  even  though  it  is  a 
"relationship  of  avoidance,"  demands  such  a  connection.  But 
such  a  "relationship  of  avoidance" .  loses  its  force  if  the  cause 
lies  within  our  own  system.  Such  a  cause  is  rendered  possible 
of  acceptance  for  our  own  galaxy,  regarded  as  a  spiral,  in  the 
phenomenon  of  occulting  matter  seen  in  so  many  edgewise  or 
nearly  edgewise  spirals. 

It  is  perhaps  worth  while  here,  at  the  risk  of  some  repeti- 
tion, to  summarize  the  arguments  bearing  on  the  place  of  the 
spirals  in  cosmogony. 

A.  Regarded  as  members  of  our  own  galaxy. 

1.  They  must  be  relatively  close;  all  evidence  is 

against  this. 

2.  Spectrum  difficult  of  explanation. 

3.  No  reason  can  be  assigned  for  their  apparent 

avoidance  of  the  Milky  Way  regions. 

4.  It  is  difficult  to  place  them  in  any  scheme  of 

stellar  evolution ;  they  are  never  found  where 
the  stars  are  thickest. 

5.  Their  tremendous  speeds  place  them  in  a  class 

apart  from  all  other  galactic  objects. 

B.  Regarded  as  separate  galaxies  (island  universes). 

1.  They    are    probably    from    ten    million    to    one 

hundred  million  light-years  distant;  this  is 
in  accord  with  the  negative  results  thus  far 
secured  for  their  distances,  proper  motions, 
and  apparent  rotation. 

2.  The  spectrum  appears  to  be  about  what  would 

be  expected  for  a  vast  congeries  of  stars. 

3.  Their   apparent   grouping   at   the   poles   of  the 

Milky  Way,  and  their  avoidance  of  its 
plane,  appear  reasonable,  if  we  assume  oc- 
culting matter  in  the  peripheral  regions  of 
our  own  galaxy  similar  to  that  seen  in  so 
many  spirals,  which  would  serve  to  cut  off 
from  our  view  spirals  near  the  plane  of  the 
Milky  Way. 


THE  NEBULAE  109 

4.  The  occurrence  of  novae  in  the  spirals  would  be 

expected  if  the  spirals  are  individual  galaxies. 

5.  Their  great  velocities  are  less  difficult  of  explana- 

tion, and  accord  well  in  order  of  magnitude 
with  those  found  for  the  Magellanic 
Clouds,  which  may  perhaps  be  similar 
structures,  relatively  close  to  us. 

It  is  certainly  a  wonderful,  a  brain-staggering  conception, 
more  tremendous  even  than  any  other  of  the  mighty  ideas  and 
facts  of  astronomy,  that  our  own  stellar  universe  may  be  but 
one  of  hundreds  of  thousands  of  similar  universes.  It  is  a 
familiar  saying  that,  "An  undevout  astronomer  is  mad".  This 
can  not  be  interpreted  too  literally ;  there  are  many  astronomers 
who  are  certainly  not  mad,  but  who  could  not,  by  any  stretch 
of  the  imagination,  be  termed  devout,  in  the  ordinary 
acceptance  of  that  term.  But,  in  a  larger  sense,  the  saying  is  a 
true  one.  Familiarity  with  these  mighty  concepts  most 
certainly  does  not  breed  contempt,  does  not  dull  our  awe  at  the 
mightiness  of  the  universe  in  which  we  play  so  small  a  part. 
It  is  very  doubtful  if  any  of  those  who  are  seriously  studying 
the  heavens  ever  lose  their  feeling  of  reverence  for  this 
supremely  wonderful  universe  and  for  Whoever  or  Whatever 
must  be  behind  it  all. 


ASTRONOMICAL  DISCOVERY1 

By  HEBER  D.  CURTIS 

The  usefulness  of  a  science  to  the  world,  and  its  intrinsic 
value  as  a  pure  science,  are  not  necessarily  measured  by  its 
capacity  for  growth,  nor  by  the  number  and  the  relative 
importance  of  the  discoveries  which  can  be  credited  to  its 
pursuit.  But  our  estimate  of  its  vitality,  its  charm  as  a  field 
of  research,  and  that  allurement  which  the  scientifically 
militant  mind  feels  in  the  presence  of  problems  awaiting  solu- 
tion, are  all  in  great  measure  dependent  upon  such  considera- 
tions. 

It  is  not  easy  to  make  a  definite  and  precise  statement  which 
shall  include  all  the  elements  entering  into  a  discovery.  Into 
the  detection  of  a  new  truth  there  may  enter  any  or  all  of  such 
factors  as  increased  instrumental  equipment,  refined  methods 
of  manipulation,  more  powerful  analytical  processes,  patience 
and  perseverance,  pure  inspiration,  or  even  pure  chance.  The 
discovery  of  the  sun-spots,  or  that  of  the  companion  to  Sirius, 
may  be  assigned  as  a  direct  result  of  the  use  of  new  or 
improved  apparatus;  the  first  stellar  parallax  was  primarily 
due  to  Bessel's  manipulative  skill;  fifteen  years  of  patient 
search  was  involved  in  the  discovery  of  the  asteroid  Astraea ; 
Keeler's  work  on  Saturn's  rings  may  well  be  regarded  as  pure 
inspiration  combined  with  great  technical  skill;  the  discovery 
of  Uranus  was  largely  chance.  ".  .  .  the  only  safe  con- 
clusion seems  to  be  that  there  are  no  general  rules  of  conduct 
for  discovery."  (Turner) 

It  would  be  quite  possible,  then,  to  limit  our  treatment  of 
the  subject  exclusively  to  instruments,  the  purely  mechanical 
adjuncts  of  discovery,  and  to  describe  the  improvements  in  our 
telescopes,  meridian  circles,  zenith  telescopes,  micrometers, 
cameras,  spectrographs,  and  photometers.  Such  a  view-point, 
though  partial  and  inadequate,  would  be  a  legitimate  one,  for 
certainly,  in  the  final  analysis,  all  astronomical  discovery 


Delivered  April  6,   1917. 


PLATE  XXV.    THE  36-lNCH  REFRACTOR,  LICK  OBSERVATORY. 


ASTRONOMICAL  DISCOVERY  111 

depends  upon  such  tools.  Without  the  telescope,  astronomy 
could  have  advanced  but  little  beyond  the  pre-Galilean  epoch. 
Science,  like  civilization  itself,  is  a  matter  of  tools. 

It  would  be  equally  permissible  to  approach  the  subject 
from  the  standpoint  of  processes,  emphasizing  the  astronomer's 
methods  of  handling  his  tools,  rather  than  the  tools  themselves. 
In  such  a  treatment  there  would  be  involved  a  discussion  of 
the  accuracy  necessary  in  astronomical  processes,  the  search 
for  minute  sources  of  error,  the  methods  of  measuring  exceed- 
ingly small  quantities,  and  the  patient  accumulation  of  details. 
This  course  would  lead  directly  to  a  consideration  of  the  nature 
of  an  astronomer's  work,  as  an  element  in  discovery.  The 
objection  may  perhaps  be  raised  that  the  routine  of  astronom- 
ical work  has  little  to  do  with  discovery.  The  objection  is 
not  a  valid  one.  As  a  matter  of  fact,  the  work  of  the  scientist 
is  discovery,  as  closely  as  it  is  possible  to  define  so  elusive  an 
entity.  Discovery  is  coterminous  with  scientific  work;  all 
research  has  discovery  for  its  aim,  if  not  for  its  result. 
Discovery  is  merely  scientific  work  which  has  the  good 
fortune  to  produce  definite  results. 

I  have  preferred  not  to  limit  myself  to  either  of  these  points 
of  view,  though  utilizing  some  material  from  each  one.  There 
are  certain  well-marked  differences  in  the  older  and  the  more 
modern  epochs  of  astronomical  discovery,  characteristic  of 
other  sciences  as  well.  The  attempt  will  be  made  to  compare 
and  to  contrast  the  methods  of  astronomical  discovery 
developed  in  the  past  half-century  with  those  o£  earlier  dates. 
Earlier  discovery  was  generally,  though  not  invariably,  that  of 
an  isolated  fact.  The  modern  epoch  has  produced  a  vast 
number  of  individual  discoveries,  but  counts  its  real  progress 
in  wider  generalizations  deduced  from  large  numbers  of 
separate  discoveries;  it  deals  with  classes  of  objects,  rather 
than  with  imits. 

For  the  nature  of  our  science,  its  overwhelming  subject- 
matter,  has  served  to  make  modern  astronomical  discovery  a 
process  requiring  the  cooperation  of  many  observers,  or  even 
many  observatories,  before  sufficient  evidence  can  be  secured 
for  some  sweeping  deduction  which  may  rank  as  a  discovery 
relating  to  a  class,  based  upon  hundreds  of  discoveries  on  the 


112  THE  ADOLFO  STAHL  LECTURES 

units  of  that  class.  The  complexity  and  richness  of  our  raw 
material  becomes,  to  some  extent,  a  disadvantage.  What  is  the 
goal  of  astronomical  discovery?  In  the  widest  sense,  its  pur- 
pose is  to  find  out  all  that  is  possible  about  the  universe,  the 
mighty  scheme  that  lies  beneath  it  all,  how  came  our  Sun,  the 
planets,  and  the  stars  into  existence,  what  has  been  our  past, 
and  what  is  our  probable  future  history.  But  such  observa- 
tions on  a  thousand  million  stars  and  nebulae  would  mean  a 
program  of  work  which  is  practically  infinite,  beyond  the 
powers  of  all  the  observatories  in  the  world,  though  they 
worked  at  the  problem  for  thousands  of  years.  All  that 
astronomy  can  hope  to  do,  on  the  observational  side,  is  to 
secure  the  fundamental  facts  for  as  many  representative  objects 
as  possible,  and  to  fit  these  facts  into  a  coherent  evolutionary 
scheme. 

There  has  been  a  tendency  to  underrate  the  apparently 
minute  advances  buried  in  the  files  of  modern  technical  litera- 
ture when  such  researches  are  compared  with  the  confident, 
mighty  journeys  into  unknown  fields  made  by  the  pioneers  of 
scientific  discovery.  It  is  true  that  much  of  modern  scientific 
progress  has  resulted  from  a  consideration  of  minute  residual 
phenomena,  or  has  depended  upon  measured  quantities  but 
little  larger  than  the  probable  error  of  the  methods  employed. 
Argon,  xenon,  and  neon,  existing  in  our  atmosphere  in  very 
small  quantities,  and  the  variation  of  latitude,  form  excellent 
examples  of  such  "residual"  discoveries.  The  radial  velocities 
of  the  stars,  aad  their  distances,  are  determined  by  the  measure- 
ment of  exceedingly  small  quantities. 

It  is  perhaps  for  such  reasons  that  a  mere  superficial  com- 
parison of  past  and  present  advances  in  many  fields  of  scientific 
endeavor  is  apt  to  leave  the  impression  that  modern  research 
is  a  matter  involving  merely  minute  technicalities  and  the 
accumulation  of  statistical  detail.  Can  modern  astronomy,  for 
example,  afford  instances  of  inventions  or  discoveries  which 
can  parallel  such  ten-league  strides  as  were  made  by  the 
invention  of  the  telescope,  the  recognition  of  the  law  of  gravita- 
tion, the  finding  of  Jupiter's  four  major  satellites,  or  the 
detection  of  the  new  planets  Uranus  and  Neptune  ? 

That  the  goal  of  a  complete  and  perfect  knowledge  is 


ASTRONOMICAL  DISCOVERY  113 

unattainable  is  accepted  as  axiomatic;  in  the  words  of  Tenny- 

Yet  all  experience  is  an  arch, 

Wherethrough  gleams  that  untraveled  world,  whose  margin  fades, 
Forever  and  forever  as  I  move. 

But  is  the  rapidity  of  scientific  discovery  gradually  diminish- 
ing, approaching  asymptotically  to  the  limit  of  a  perfect  knowl- 
edge ?  Any  adequate  analysis  of  recent  progress  in  any  branch 
of  science  will  indicate  to  the  thoughtful  observer  that,  so  far 
from  any  approach  to  the  stagnation  of  perfection,  there  never 
has  been  an  epoch  when  scientific  advancement  has  been  more 
rapid,  never  a  period  of  more  revolutionary  progress.  Three 
decades  ago  there  were  many  who  felt  that  physics  and 
inorganic  chemistry  were  fast  crystallizing  into  a  form  which, 
like  trigonometry,  might  reasonably  be  regarded  as  approxi- 
mately final ;  the  permanent  foundations  had  been  laid.  Small 
changes  were  naturally  to  be  expected  in  the  superstructure, 
but  such  fundamentals  as  the  form  of  the  atomic  theory  then 
current,  the  permanence  of  the  chemical  elements,  and  the 
invariability  of  the  atomic  weights  were,  like  the  atoms  them- 
selves, unchangeable  entities.  What  chemist  or  physicist  of 
that  day  could  have  imagined  the  annihilation  of  such  basic 
hypotheses,  could  have  conceived  of  the  transmutation  of  the 
elements  occurring  in  radioactive  processes,  or  could  have 
admitted  the  existence  of  iso topic  forms  of  lead,  identical  in 
chemical  constitution,  but  differing  in  atomic  weight  in  accord- 
ance with  the  radioactive  parent  metals  of  which  the  leads  are 
the  eternally-old  residue,  still  being  created? 

In  considering,  more  in  detail,  the  elements  which  enter  into 
the  processes  of  astronomical  discovery,  one  must  guard  against 
certain  all  too  prevalent  misconceptions  of  the  character  of  the 
work  of  the  astronomer.  The  popular  idea  of  an  astronomer 
is  that  of  a  man  sitting  at  one  end  of  a  telescope;  the  larger 
the  telescope,  the  greater  the  astronomer;  he  is  looking  at 
something  very  interesting,  preferably  the  planet  Mars,  and 
discoveries  follow  thick  and  fast.  Nothing  could  well  be 
further  from  the  truth.  Such  misconceptions  exclude,  as  well, 
those  manifold  astronomical  activities  which  depend  less 
directly  upon  observation,  but  are  at  least  of  equal  value,  and 
have  given  rise  to  discoveries  of  the  highest  importance. 


114  THE  ADOLFO  STAHL  LECTURES 

For  our  sciences,  not  only  as  such,  but  in  their  internal 
development  as  well,  form  no  exception  to  the  familiar  truth 
that  this  is  an  age  of  specialization.  We  no  longer  find  men 
who  will  pretend  to  the  entire  sum  of  human  knowledge,  as 
did  the  scholastics  of  the  Middle  Ages;  it  was  then  actually 
possible  for  one  brain  to  compass  practically  the  entire  store 
of  accumulated  fact.  It  has  been  said  that  with  the  death  of 
Sylvester  there  passed  away. the  last  man  who  could  with  some 
reason  be  said  to  be  thoroughly  conversant  with  all  depart- 
ments of  the  one  science  of  mathematics,  as  it  existed  in  his 
day.  So  much  has  the  field  of  this  mighty  science  expanded 
since  his  time  that  it  is  now  absolutely  impossible  for  any 
single  intellect  to  encompass  the  whole.  Similarly,  the  chemist, 
the  physicist,  the  biologist,  or  the  engineer,  must  now  perforce 
limit  his  mastery  of  his  science,  and  the  field  of  his  creative 
work,  to  some  comparatively  limited  sub-province.  One 
astronomer  may  devote  his  entire  life  to  the  measurement  of 
double  stars,  another  to  the  determination  of  accurate  stellar 
positions,  another  to  the  analysis  of  their  light,  still  another  to 
photography.  One  very  important  class  rarely  make  a  direct 
observation  or  look  through  a  telescope,  but  carry  out  the 
extensive  computations  or  the  mathematical  investigations, 
without  which  analytical  sifting  process  the  multitudinous 
observations  of  the  practical  astronomer  would  sometimes 
have  little  value  for  the  progress  of  the  science.  Of  this  type 
were  such  men  as  Hansen,  Poincare,  and  Hill,  honored  for 
their  mathematical  discoveries,  though  their  names  may  not  be 
found  in  the  more  elementary  astronomical  text-books.  Hill 
once  stated  that  the  twenty  years  of  continuous  work  which 
Delaunay  had  devoted  to  the  lunar  theory  was  undoubtedly 
the  greatest  task  one  man  had  ever  carried  through  single- 
handed;  with  the  simple,  unegotistic  certainty  of  genius  he 
added  that  he  would  give  second  place  to  the  fourteen  years 
which  he  himself  had  devoted  to  the  mathematical  theory  of 
the  motions  of  Jupiter  and  Saturn.  Such  men  must  surely 
rank  as  discoverers. 

What  is  it,  then,  which  constitutes  a  discovery,  and  how 
may  it  be  defined?  Can  a  distinction  be  made  between  Bond's 
discovery  of  the  "crepe"  ring  of  Saturn  (made  on  a  night  when 


PLATE  XXVI.    THE  37-lNcn  MILLS  REFLECTOR,  SANTIAGO,  CHILE. 


ASTRONOMICAL  DISCOVERY  115 

haze  obliterated  all  but  the  brighter  stars!)  and  Euler's  equa- 
tion, which  gives  a  remarkable  relation  between  the  times,  the 
distances  from  the  central  body,  and  the  length  of  the  arc 
traversed  in  a  parabolic  orbit  under  the  law  of  gravitation,2  of 
the  first  importance  in  certain  orbit  computations?  Can 
relative  values  be  assigned  to  Herschel's  discovery  of  Uranus, 
and  to  the  method  of  determining  stellar  velocities  in  the  line 
of  sight  by  the  Doppler-Fizeau  shift  of  the  spectral  lines,  which 
forms  one  of  the  most  powerful  methods  of  modern  astronomy? 
Such  comparisons  are  futile;  the  race  of  discoverers  is 
rather  to  be  regarded  as  a  pure  democracy,  where  all  have 
equal  rank.  Every  advance  has  its  value  in  contributing  to 
our  knowledge  of  the  whole,  whether  the  discovery  is  that  of 
a  comet  or  double  star,  a  more  powerful  method  of  analysis  of 
planetary  motions,  or  a  masterly  deduction  of  evolutionary 
processes  derived  from  a  consideration  of  thousands  of  isolated 
facts.  It  is  true  that  the  effort  involved,  and  the  ability 
required,  in  different  astronomical  discoveries  are  sometimes  so 
unequal  that  it  seems  impossible  to  give  the  results  an  equal 
weight.  Given  the  invention  of  the  telescope,  and  it  would 
appear  that  the  moons  of  Jupiter,  the  craters  on  the  Moon,  and 
the  spots  on  the  Sun,  must  be  found  immediately,  as  indeed 
they  were ;  a  bright  comet  or  nova  is  frequently  "discovered" 
by  scores.  On  the  other  hand,  it  has  taken  several  generations 
to  accumulate  such  a  knowledge  of  stellar  positions  and  motions 
as  to  make  possible  the  two-stream  theory  of  star  drift. 

Whatever  may  be  the  differences  in  the  older  and  the  more 
recent  fields  of  astronomical  discovery,  it  will  be  found  that 
one  factor,  and  that  the  most  important  one  of  all,  is  the  same 
in  all  present-day  researches  in  any  science  as  it  was  of  old, 
and  that  is  the  element  of  unlimited  patience,  and  close, 
unwearied  application.  Darwin  put  the  secret  very  clearly, 
when  he  stated  that  his  successes  were  chiefly  due  "to  the 
love  of  science,  unbounded  patience  in  long  reflecting  over  any 
subject,  industry  in  observing  and  collecting  facts,  and  a  fair 
share  of  invention  as  well  as  of  common  sense." 

A  few  typical  instances  will  serve  to  illustrate  the  older 
field  of  more  purely  individual  discovery. 

2  This  equation,   in   mathematical   symbols,   takes  the   form, — 

6fc  (*-t      =     r    +  r    +  s)%  ±     r    +  r    -  j% 


116  THE  ADOLFO  STAHL  LECTURES 

Herschel,  in  the  course  of  his  systematic  survey  of  the  sky, 
was  one  night  surprised  to  find  an  object  which  seemed 
radically  different  from  a  star,  in  that  its  apparent  size 
increased  as  he  applied  successively  higher  magnifications.  He 
does  not  appear  to  have  recognized  an  actual  disk,  but  he 
satisfied  himself  that  the  object  was  not  a  star;  he  made  care- 
ful record  of  the  observation  and  the  position  of  the  suspected 
object,  and  was  able  later  to  note  that  it  had  moved.  He 
announced  the  discovery  as  a  comet.  It  was  some  time  before 
it  was  realized  that,  instead  of  a  comet,  he  had  found  a  new 
major  planet !  Herschel  named  it  Georgium  Sidus,  the 
Georgian  star,  in  honor  of  his  royal  patron,  and  it  was  later 
known  as  Herschel.  Many  years  later  the  more  fitting  name, 
Uranus,  supplanted  both  earlier  designations.  This  discovery 
has  often  been  referred  to  as  one  in  which  pure  chance  played 
a  very  large  part;  it  has  been  described  as  "the  finding  of 
something  by  an  observer  who  was  looking  for  anything." 
The  discovery  was  the  result,  however,  of  a  definite  and  well- 
planned  program  of  work.  To  make  observations  carefully,  to 
keep  accurate  records,  to  leave  nothing  to  chance,  and  to  pass 
over  no  point  of  difference  without  seeking  a  reason  for  the 
discrepancy,  are  all  fundamental  factors  in  scientific  discovery, 
and  none  of  these  essentials  was  omitted  by  Herschel.  As  a 
matter  of  fact,  it  has  since  been  found  that  Uranus  had 
previously  been  observed  no  fewer  than  seventeen  times  on 
various  meridian-circle  programs,  the  first  recorded  observation 
having  been  made  in  1690,  a  century  before  Herschel's  time. 

In  the  early  part  of  the  nineteenth  century  there  was  a 
janitor  at  the  Observatory  of  Marseilles  who  became  deeply 
interested  in  astronomy;  he  taught  himself  all  he  could,  and 
commenced  to  devote  himself  with  unremitting  patience  to  one 
very  definite  field  of  discovery.  He  found  thirty-seven  comets 
during  his  life,  and  rose  to  the  position  of  director  in  another 
observatory.  Through  these  cometary  discoveries  the  name 
of  Pons  is  today  a  fairly  familiar  one  in  astronomical  literature, 
though  there  are  few  astronomers  who  could  name  the  director 
at  Marseilles  under  whom  Pons,  as  a  janitor,  started  his  astro- 
nomical career. 

Again,  a  country  apothecary  in  a  little  German  town  began 


ASTRONOMICAL  DISCOVERY  117 

to  watch,  to  sketch,  and  to  keep  an  accurate  record  of  sun- 
spots.  He  was  provided  with  a  little  telescope  but  slightly  more 
powerful  than  some  modern  binoculars.  He  kept  assiduously 
at  his  self-appointed  task  for  many  years,  and  finally  was 
enabled  to  announce  from  his  own  records,  and  to  corroborate 
by  analysis  of  older  observations,  that  the  sun-spots  have  a 
regular  period,  occurring  in  far  greater  numbers  about  every 
eleven  years.  To  Schwabe,  the  country  apothecary,  and  not 
to  any  of  the  more  powerfully  equipped  observatories  of  his 
time,  belongs  the  honor  of  the  discovery  of  this  fundamental 
solar  phenomenon. 

Many  similar  examples  might  be  given  of  astronomical 
researches  carried  out  with  a  minimum  of  instrumental  equip- 
ment, plus  a  maximum  of  energy,  patience,  and  manipulative 
skill.  We  recall  how  Barnard  set  up  his  little  telescope,  and 
commenced  the  untiring  observational  toil  which  has  made 
him  a  valued  member  of  the  staff  of  two  of  America's  largest 
observatories.  Burnham,  a  reporter  in  the  Chicago  law-courts, 
bought  himself  a  six-inch  lens,  which  he  mounted  at  the  rear 
of  his  home.  After  his  day's  work,  he  spent  most  of  his  nights 
in  observing  and  discovering  double  stars,  becoming  the 
world's  leading  authority  in  that  field. 

Telescopes, — even  small  ones, — are  by  no  means  a  pre- 
requisite in  certain  types  of  astronomical  discovery.  A  little 
before  the  middle  of  the  last  century  astronomers  were  puzzled 
by  the  fact  that  Uranus,  then  the  outermost  planet  known,  was 
deviating  slightly  from  the  path  which  computation  had 
indicated  for  it.  It  was  "out"  from  its  computed  position  by 
about  the  angle  subtended  by  the  dots  over  the  letter  i  on  this 
page,  when  viewed  from  a  distance  of  two  feet;  a  seemingly 
minute  discrepancy,  but  intolerable  from  the  standpoint  of 
accurate  gravitational  theory.  Independently,  and  almost 
simultaneously,  two  men  set  themselves  at  the  task  of  finding 
the  reason  for  this  irregularity.  One  of  these  was  Adams,  an 
Englishman  of  twenty-five,  just  out  of  college.  The  other, 
Leverrier,  was  a  brilliant  Frenchman  of  thirty-five,  who  had 
planned  for  a  career  in  the  French  state  tobacco  administra- 
tion ;  to  this  end  he  had  specialized  in  chemistry,  but  he  gave  up 
this  field  when  offered  an  opportunity  to  become  a  teacher  of 


US.  THE  ADOLFO  STAHL  LECTURES 

astronomy.  It  would  be  impossible  to  give  a  non-technical 
outline  of  the  intricate  calculations  which  these  men  carried 
through  independently,  to  the  same  conclusion,  that  Uranus 
was  being  pulled  slightly  out  from  its  predicted  path  by  a  still 
more  distant  planet,  and  they  indicated  the  region  where 
search  should  be  made  for  this  disturbing  body.  It  is  worth 
emphasizing,  at  this  point,  that  neither  of  these  men  had  tele- 
scopes, so  they  communicated  their  predictions  to  other 
astronomers.  As  a  result,  Neptune  was  found  fairly  close  to 
the  place  calculated,  perhaps  the  most  notable  instance  of 
discovery  in  the  annals  of  mathematical  astronomy. 

Great  as  have  been  the  prizes  won  by  the  discoveries  of 
astronomy's  earlier  history,  the  present  epoch  is  not  only  far 
more  prolific,  but  intrinsically  richer.  In  one  sense,  it  is  super- 
fluous to  ask  how  discoveries  are  made;  they  are  simply  the 
results  of  scientific  work,  which  in  its  turn  means  merely  the 
using  of  the  tools  of  a  science  upon  objects  or  facts  hitherto 
unexamined  or  unexplained.  The  more  powerful  our  tools, 
the  faster  will  be  our  rate  of  work  and  discovery.  If  the 
attempt  be  made  to  define  and  to  analyze  the  more  important 
factors  contributing  to  the  present  state  of  astronomical 
discovery,  so  rich  in  both  quantity  and  quality,  we  shall  find 
several  contributing  causes,  interdependent,  rather  than 
separate  in  their  effects.  Though  it  will  be  of  interest  to 
segregate  these,  it  will  be  seen  at  once  that  it  is  really  possible 
to  combine  all  in  the  one  phrase, — better  tools.  These  factors 
are: 

1.  The  existence  of  many  large  observatories  liberally  sup- 
ported   by    government    appropriations    or    by    benefactions, 
devoted   almost   exclusively   to   astronomical   research.     The 
number  of  investigators,  and  the  available  instrumental  equip- 
ment, are  immeasurably  greater  than  they  were  half  a  century 
ago. 

2.  The  great  advances  in  mechanical  processes  which  have 
enabled    the    instrument-maker    to    furnish    more    accurate 
instruments   and   larger   lenses   and   mirrors ;   here   we   must 
include  the  work  of  the  engineer  in  providing  adequate  mount- 
ings for  giant  instruments. 

3.  A  tremendous  development  in  the  character  and  power 


PLATE  XXVII.     THE  72-lNCH  REFLECTING  TELESCOPE  OF  THE  DOMINION 

ASTROPHYSICAL    OBSERVATORY. 


ASTRONOMICAL  DISCOVERY  119 

of  the  methods  employed,  due  in  large  part  to  the  application 
and  adaptation  of  processes  perfected  in  the  allied  sciences  of 
physics  and  chemistry. 

4.  Properly  to  be  included  under  the  preceding  heading, 
but  so  important  as  to  deserve  separate  mention, — the  use  of 
photography. 

Of  the  above,  it  is  photography,  more  particularly  the  per- 
fection of  the  modern  dry  plate,  which  has  most  completely 
and  radically  changed  the  methods  of  astronomical  discovery. 
Fully  three-fourths  of  all  modern  astronomical  observations 
are  photographic,  and  it  seems  highly  probable  that  the  day 
may  come  when  practically  all  astronomical  research  will 
depend  upon  photographic  processes.  A  better  conception  of 
the  manifold  applications  of  photography  in  astronomy  may 
be  derived  from  the  following  short  summary  : 

PHOTOGRAPHY  IN  OBSERVATIONAL  ASTRONOMY 

FUNDAMENTAL  ASTRONOMY.  The  determination  of  the 
absolute  positions  of  the  Sun,  the  planets,  and  the  stars 
is  still  mainly  visual,  though  photography  is  invading 
this  field. 

THE  SUN.  Fully  90%  of  modern  observations  are  photo- 
graphic. 

THE  MOON.  Mainly  photographic,  though  for  the  study  of 
minute  details  visual  observation  has  some  advantages. 

THE  PLANETS.  Photography  as  yet  is  used  to  but  a  small 
extent ;  visual  observations  are  still  best  for  the  study  of 
surface  detail. 

THE  ASTEROIDS.  Fully  80%  is  photographic,  and  about  100% 
in  the  work  of  discovering  new  asteroids. 

THE  COMETS.  Comet  positions  are  generally  determined  visu- 
ally, but  for  studies  of  physical  structure  photography 
is  supreme. 

THE  STARS.  For  general  charting  and  mapping  purposes  visual 
observation  is  fast  being  superseded  by  photography. 

In  the  determination  of  stellar  distances  fully  75% 
is  photographic,  and  the  proportion  is  increasing. 

For    physical    constitution,    spectroscopic    studies, 
radial  velocities,  etc.,  practically  100%  photographic. 


120  THE  ADOLFO  STAHL  LECTURES 

THE  VARIABLE  STARS.  Possibly  50%  photographic;  visual 
observations  still  of  great  value ;  perhaps  eventually 
entirely  photographic  or  photo-electric. 

THE  DOUBLE  STARS.  Still  nearly  100%  visual;  a  field  in  which 
photography  seems  as  yet  to  have  little  chance  for  suc- 
cessful competition.  ^ 

THE  NEBULAE.    100%  photographic. 

Illustrations  of  the  power  of  the  photographic  method 
might  be  multiplied  almost  indefinitely,  but  such  a  course  would 
far  exceed  the  limits  of  a  single  lecture.  A  few  instances  must 
suffice. 

Fig.  1,  Plate  XXVIII,  shows  a  small  section,  about  eight 
by  ten  degrees  in  size,  of  a  chart  of  the  Banner  Durchmus- 
terung,  a  great  star  map  which  was  made  about  the  middle  of 
the  nineteenth  century  and  is  still  an  indispensable  aid  to  the 
practical  astronomer.  The  entire  set  of  charts  includes  about 
324,000  stars  of  magnitude  9.5  or  brighter,  in  that  part  of  the 
sky  included  between  the  north  celestial  pole  and  twenty-one 
degrees  south  declination;  the  positions  were  determined  with 
a  small  telescope  of  about  three  inches  aperture.  The  observa- 
tions, and  the  preparation  of  these  charts  and  the  accompany- 
ing catalogue,  meant  a  vast  amount  of  patient  work.  At  the 
center  of  the  figure  is  seen  a  small  dotted  area,  which  indicates 
the  position  of  one  of  the  largest  and  brightest  of  the  spiral 
nebulae,  Messier  33,  which  is  bright  enough  to  be  made  out  in 
the  small  telescope  used  for  the  survey.  In  Fig.  2  on  the 
same  plate,  is  shown  the  same  region,  on  the  same  scale,  as 
photographed  with  the  Crocker  Photographic  telescope  at  Lick 
Observatory.  It  will  be  noticed  that  the  nebula  is  not  much 
more  in  evidence  than  on  the  star  chart,  but  it  would  be  difficult 
to  say  by  how  many  times  we  must  multiply  the  number  of 
stars  shown  on  the  chart  to  obtain  the  total  registered  in  the 
photograph.  Moreover,  the  photographic  plate  was  secured  in 
a  few  hours,  as  against  many  nights  of  work  in  the  case  of  the 
star  map,  and  it  is  a  permanent  record,  available  for  study  and 
measurement  as  long  as  the  photographic  film  shall  endure.  In 
the  star  chart,  the  human  element  has  entered  into  every  step 
of  the  process  from  the  observation  of  the  individual  stars  to 


FIG.  1— B.  D.  Chart,  M.  33  Central. 


FIG.  2— Crocker  telescope  photograph,  M.  33  Central. 
PLATE  XXVIII 


ASTRONOMICAL  DISCOVERY  121 

the  engraving  of  the  map ;  at  no  time  in  the  production  of  the 
second  illustration  has  any  manual  dexterity  or  elaborate 
calculation  entered  directly  into  the  final  record  of  the  relative 
position  of  an  individual  star.  With  a  telescope  of  greater 
focal  length  we  can  secure  a  much  larger-scale  photograph  of 
any  desired  small  area  of  the  region.  Plate  XXIX  shows  the 
central  nebula,  as  photographed  with  the  Crossley  reflector.3 

Great  as  has  been  the  enrichment  of  the  field  of  astronom- 
ical discovery  caused  by  the  introduction  of  the  photographic 
method,  that  due  to  the  spectroscope  is  even  greater.  Prior  to 
the  development  of  the  spectrographic  method,  the  story  told 
by  the  ray  of  light  from  a  distant  star  was  a  comparatively 
brief  one.  The  ray  of  light  told  us  little  more  than  the 
precise  position  of  the  star,  whether  the  star  was  variable,  and 
whether  the  star  was  double ;  it  gave  us  no  information  what- 
ever as  to  what  the  star  was.  We  know  now  that  there  lie 
buried  within  the  inconceivably  rapid  vibration  complex  of  the 
light  ray  whole  volumes  of  information  with  regard  to  the 
temperature,  physical  condition,  and  chemical  constitution  of 
the  star.  The  information  may  have  been  ten  thousand  or  more 
years  on  the  road,  and  we  know  nothing  as  yet  of  the  wonder- 
ful medium  through  which  the  record  of  the  light  vibrations 
is  transmitted  to  us,  but  we  can  analyze  these  vibrations  and 
read  a  part  of  their  message  merely  by  passing  the  light 
through  the  prisms  of  a  spectrograph.  The  spectrograph  has 
changed  the  entire  content  of  astronomical  discovery  from  a 
record  of  position  and  movement  to  a  record  of  composition 
and  quality. 

It  has  been  stated  earlier  that  the  trend  of  the  older  field 
of  astronomical  research  was  toward  the  individual  discovery, 
while  that  of  the  modern  field  is  toward  the  class  or  group,  the 
super-discovery  based  upon  hundreds  of  individual  advances. 
The  discovery  of  the  first  asteroid,  the  first  double  star,  the  first 
spectroscopic  binary,  were  events  of  great  astronomical  impor- 
tance. Scores  of  these  objects  are  now  found  yearly,  and  their 
utility  depends  less  on  their  intrinsic  values  as  separate  facts 

3  At  this  point  in  the  lecture,  as  delivered,  many  lantern  slides  were  shown 
comparing  the  older  drawings  of  sun-spots,  comets,  the  Moon,  and  nebulae,  with 
modern  photographs.  Slides  were  shown  also  illustrating  the  photographic 
discovery  of  asteroids  and  of  the  Ninth  Satellite  of  Jupiter,  and  the  photography 
of  stellar  spectra. 


122  THE  ADOLFO  STAHL  LECTURES 

than  upon  the  larger  generalizations  which  may  be  drawn  from 
the  group. 

Recent  astronomical  history  is,  however,  not  without  many 
instances  of  brilliant  individual  discoveries.  It  has  added  to 
the  older  record : — two  satellites  of  Mars,  five  of  Jupiter,  one 
of  Saturn,  hundreds  of  asteroids,  hundreds  of  variable  stars, 
thousands  of  visual  and  spectroscopic  binaries,  hundreds  of 
thousands  of  nebulae,  millions  of  stars,  and  a  multitude  of  facts 
bearing  on  the  physical  constitution  of  the  Sun,  the  stars,  and 
the  nebulae.  It  will  be  more  representative  of  modern  prog- 
ress, however,  to  give  a  resume  of  the  results  from  a  single 
method  of  research,  rather  than  to  limit  ourselves  to  individual 
instances ;  the  field  which  will  be  briefly  treated  is  that  involv- 
ing the  determination  of  stellar  velocities  by  the  Doppler- 
Fizeau  shift  of  the  spectral  lines.  The  results  of  this  method 
of  attack  have  involved  a  host  of  minor  discoveries,  and  have 
thrown  a  flood  of  light  upon  many  of  the  problems  which  the 
origin  and  evolution  of  the  universe  present  to  the  mind  of 
man.  These  researches,  moreover,  are  typical  of  modern  prog- 
ress in  that  they  have  involved  the  cooperation  of  many 
workers,  and  have  necessitated  years  of  observation  before 
sufficient  data  could  be  secured  to  make  possible  the  sifting 
out  and  elucidation  of  the  basal  principles. 

We  are  all  familiar  with  the  fact  that  a  musical  note  is 
said  to  have  a  certain  pitch ;  that  the  reason  why  one  note  has 
a  higher  pitch  of  sound  than  another  is  because  it  is  due  to  a 
greater  number  of  air  waves  or  vibrations  per  second. 
Similarly  we  may  think  of  light  as  possessing  something  that 
is  closely  analogous  to  pitch  in  sound.  Blue  light,  for  instance, 
has  about  twice  as  many  light-waves  per  second  as  red  light 
(eight  hundred  trillion  for  blue,  four  hundred  trillion  for  red 
light)  so  we  may  say,  for  the  purposes  of  illustration,  that 
blue  light  is  about  an  "octave"  higher  in  pitch  than  red  light. 
Some  of  you  may  perhaps  have  been  on  a  railroad  train  when 
another  train  was  passing  in  the  opposite  direction,  and  sound- 
ing its  whistle  as  it  passed.  On  such  an  occasion  it  takes  only 
a  moderately  keen  ear  to  notice  that  the  pitch  of  the  passing 
locomotive's  whistle  is  half  a  tone  or  so  higher  as  it  is  approach- 
ing, and  suffers  a  similar  drop  in  pitch  as  it  recedes.  The  speed 


PLATE    XXIX.     M.  33  TRIANGULI. 
Photographed  by  J.  E.  Keeler,  Crossley  Reflector,  Sept.  12,  1899. 


ASTRONOMICAL  DISCOVERY  123 

at  which  the  trains  are  moving  makes  the  pitch  of  the  sound 
higher  when  they  are  approaching,  and  lower  when  they  are 
receding  from  each  other.  Similarly,  if  a  star  is  coming 
swiftly  toward  us,  or  moving  swiftly  away  from  us,  this  veloc- 
ity changes  the  "pitch"  of  the  light  by  a  small  amount.  We 
can  measure  the  amount  of  this  shift  in  light-pitch  or  wave- 
length if  we  pass  the  light  through  the  prisms  of  a  spectro- 
graph,  using  the  light  from  some  terrestrial,  stationary  source 
for  comparison  to  give  us  our  "zero  point".  We  can  thus 
determine  the  speed  of  the  star  directly  tozvard  or  directly  away 
from  us,  known  as  its  radial  velocity,  in  miles  per  second.  This 
principle  was,  in  itself,  a  notable  discovery,  but  the  discoveries 
which  have  followed  as  by-products  of  the  application  of  this 
method  to  the  determination  of  the  radial  velocities  of  many 
celestial  objects  form  a  very  important  part  of  recent  astro- 
nomical progress. 

First, — it  was  found  that  the  stars  in  one  part  of  space 
were,  on  the  average,  coming  toward  us  at  the  rate  of  twelve 
and  one-half  miles  per  second,  and  apparently  receding  at  the 
same  speed  in  the  opposite  quarter.  That  is  to  say, — the  Sun 
and  all  his  system  is  really  moving  through  space  at  the  rate 
of  about  twelve  and  one-half  miles  per  second.  This  had  been 
known  earlier  qualitatively,  but  not  quantitatively.  With  this 
came  the  knowledge  that  all  the  stars  are  in  rapid  motion,  at 
average  speeds  of  from  eight  to  twenty-one  miles  per  second, 
while  some  few  stars  are  traveling  at  speeds  of  one  hundred 
or  more  miles  per  second.  The  great  extended  nebulosities 
are  almost  at  rest  in  space;  the  planetary  nebulae  are  moving 
at  average  speeds  of  nearly  fifty  miles  per  second,  while  the 
spirals  have  the  enormous  average  speed  of  nearly  five 
hundred  miles  per  second. 

Further, — the  gradual  accumulation  of  radial  velocities  for 
many  different  stars  showed  that  the  stars  of  different  spectral 
types  were  going  at  different  average  speeds,  the  stars  which 
we  think  to  be  the  younger  moving  more  slowly,  the  older 
stars  more  rapidly.  Just  why  this  should  be  the  case  is  not  yet 
fully  understood;  some  evidence  is  now  being  accumulated 
which  may  eventually  show  that  these  systematic  differences 
in  average  speed  are  functions  of  the  masses  of  the  stars, 


124  THE  ADOLFO  STAHL  LECTURES 

rather  than  of  their  relative  positions  in  the  order  of  stellar 
age. 

In  the  progress  of  these  researches,  many  stars  were  found 
which  are  coming  toward  us  at  one  epoch,  and  receding  from 
us  at  another,  this  reversal  of  motion  recurring  at  regular 
intervals  of  time.  Such  stars  are  revolving  around  a  darker 
central  star,  which  is  generally  too  faint  to  leave  any  record 
of  its  spectrum.  They  are  known  as  spectroscopic  binaries; 
these  are  double  stars  discovered  by  the  systematic  variation 
in  the  pitch  of  the  light  they  send  to  us,  though  they  cannot  be 
seen  as  double  in  the  largest  telescopes,  because  they  are  too 
distant.  It  has  thus  been  found  that  about  one  in  every  three 
of  the  brighter  stars,  though  apparently  single  under  the 
highest  magnifying  powers,  is  a  spectroscopic  binary.  This 
discovery  has  an  important  bearing  on  all  theories  of  stellar 
evolution ;  perhaps  relatively  few  suns  have  developed  so  as  to 
have  a  retinue  of  comparatively  small  planets,  as  is  the  case  in 
our  solar  system. 

The  same  general  method  has  been  applied  with  great  suc- 
cess to  the  study  of  the  Sun  and  to  the  motions  of  the  matter 
in  and  around  the  sun-spots,  to  Saturn's  rings,  where  it  was 
shown  that  these  are  in  rotation,  but  not  solid,  and  to  many 
other  fields  of  astronomy.  Very  recently  it  has  been  found, 
in  the  same  way,  that  many  of  the  planetary  nebulae  are  in 
rapid  rotation. 

These  revolutionary  advances  in  astronomical  theory  are 
due  to  the  application  of  the  Doppler-Fizeau  principle,  with 
the  aid  of  the  spectrograph  and  the  photographic  plate,  and  are 
essentially  all  a  product  of  the  past  quarter-century.  How 
much  work  has  it  meant  for  astronomy  to  state  these  striking 
and  important  facts  as  discoveries?  Many  astronomers  and 
many  observatories  have  cooperated  in  collecting  the  necessary 
observations,  but  we  shall  consider  only  the  work  of  the  Lick 
Observatory  in  this  field. 

For  more  than  twenty  years  about  three-fourths  of  all 
the  available  time  of  the  great  refractor  has  been  devoted 
assiduously  to  the  securing  of  the  spectrographic  plates  for  this 
program,  ranging  in  exposure  times  from  a  few  minutes  in  the 
case  of  the  brightest  stars,  to  many  hours  for  fainter  objects. 


ASTRONOMICAL  DISCOVERY  125 

Nearly  eighteen  thousand  plates  have  been  taken, — the  largest 
collection  of  this  class  in  existence.  A  branch  observatory, 
that  of  the  D.  O.  Mills  Expedition  at  Santiago,  Chile,  was 
established  to  secure  these  plates  for  the  stars  in  the  southern 
skies,  inaccessible  from  our  northern  latitude.  In  the  course 
of  this  extended  program  some  twenty  people  have  taken  part 
in  the  securing  of  these  eighteen  thousand  spectrograms,  or 
have  made  the  measures  and  reductions  which  are  necessary 
before  the  velocity  of  a  star  can  be  determined  from  the 
spectrographic  negative.  Last  of  all  came  the  combination  and 
the  analysis  of  these  thousands  of  radial  velocities,  which  have 
given  rise  to  those  generalizations  of  wider  scope  resulting 
from  the  initial  discovery  of  the  Doppler-Fizeau  principle. 

It  is  with  such  extended  researches  and  larger  problems 
that  modern  astronomy  is  working,  and  the  methods  of  astro- 
nomical discovery  are  simply  the  methods  of  astronomical 
work.  In  the  larger  theory,  the  greater  truth  of  some  super- 
discovery,  there  are  frequently  combined  hundreds  of  indi- 
vidual discoveries  of  minor  rank;  for  any  fact,  previously 
unknown,  is  a  discovery. 

There  is  abundant  room  for  the  development  of  new  and 
brilliant  methods  of  attacking  such  larger  problems.  In 
general,  however,  it  appears  probable  that  future  advances 
will,  in  like  manner,  depend  upon  the  accumulation  of  many 
discoveries  concerning  the  units  of  a  class  of  objects,  and  upon 
the  careful  and  systematic  analysis  of  these  facts  for  the  basic 
truths  of  stellar  evolution. 

Little  has  been  said  as  to  that  element  of  personal  inspira- 
tion or  genius  which  enters  into  much  of  true  discovery.  It 
is  by  no  means  an  indispensable  ingredient,  iconoclastic  though 
such  a  statement  may  seen.  There  have  been  many  instances 
where  what  we  somewhat  loosely  term  "genius"  has  appeared 
to  be  the  determining  factor.  It  would  be  equally  easy  to  find 
instances  of  discovery  made  by  observers  of  mediocre  ability, 
in  which  the  "divine  fire"  was  replaced  by  mere  plodding 
patience,  or  by  the  extraneous  and  adventitious  aid  of  power- 
ful equipment.  The  machine-like  processes  of  photography 
and  spectroscopy,  the  intervention  of  the  hired  plate-measurer 
and  computer,  have  inevitably  removed  not  a  little  of  the  more 


126  THE  ADOLFO  STAHL  LECTURES 

purely  personal  element  from  the  modern  field  of  astronomical 
discovery.  Even  so,  the  personal  element  is  still  a  powerful, 
and  in  most  work,  an  essential  factor.  These  qualities  have 
been  well  summarized  by  Jevons  in  his  "Principles  of  Science"  : 

It  would  seem  as  if  the  mind  of  the  great  discoverer  must  com- 
bine contradictory  attributes.  He  must  be  fertile  in  theories  and 
hypotheses,  and  yet  full  of  facts  and  precise  results  of  experience.  He 
must  entertain  the  feeblest  analogies  and  the  merest  guesses  at  truth, 
and  yet  he  must  hold  them  as  worthless  till  they  are  verified  by  experi- 
ment. When  there  are  any  grounds  of  probability  he  must  hold  te- 
naciously to  an  old  opinion,  and  yet  he  must  be  prepared  at  any  moment 
to  relinquish  it  when  a  clearly  contradictory  fact  is  encountered. 

Though  it  seems  somewhat  paradoxical,  there  is  a  great 
deal  of  truth  in  the  saying  that  any  good  theory  brings  with 
it  more  problems  than  it  removes.  In  like  manner,  each  great 
advance  in  modern  astronomical  theory  has  brought  with  it 
a  host  of  new  problems,  and  has  opened  up  new  fields  of  vast 
extent.  We  have  seen  our  concepts  of  the  size  of  the  stellar 
universe  steadily  increase.  Where  once  we  doubtingly  dis- 
cussed distances  of  a  few  thousand  light-years,  we  now  con- 
fidently postulate  distances  of  hundreds  of  thousands  or 
millions  of  light-years.  With  the  aid  of  the  methods  con- 
tributed by  the  allied  sciences,  our  field  of  astronomical  discov- 
ery has  expanded  in  even  greater  ratio;  like  our  subject-mat- 
ter, it  is  infinite. 


PLATE    XXX.     THE    MILLS    SPECTROGRAPH    ATTACHED    TO   THE  36-lNCH 

REFRACTOR. 


IMPORTANT  EPOCHS  IN  THE  DEVELOPMENT 
OF  ASTRONOMY1 

By  R.  T.  CRAWFORD 

When  one  stands  in  awe  and  admiration  before  the 
Woolworth  building  in  New  York,  the  Campanile  of  Venice, 
or  that  of  the  University  of  California  at  Berkeley,  some 
massive  bridge  with  its  network  of  girders,  the  Milan  Cathe- 
dral, or  any  other  wonderful  work  of  man,  rarely  does  he 
consider  the  separate  and  distinct  processes  that  contribute  to 
its  construction.  It  is  the  finished  product  that  receives  the 
words  of  praise  and  commendation.  The  foundations  and 
other  parts  out  of  sight  are  almost  completely  neglected.  The 
question  "Who  was  the  architect?"  is  nearly  always  asked; 
"Who  was  the  engineer?"  is  seldom  heard.  It  is  quite  right 
that  we  should  praise  and  admire  the  designer,  but  we  should 
give  due  meed  of  praise  and  admiration  to  the  engineer  who 
figures  the  stresses  and  strains  of  the  various  members  of  the 
edifice  and  designs  the  foundations  and  without  whose  genius 
the  architect's  conception  could  not  come  to  realization.  Nor 
should  we  stop  here,  but  reserve  some  of  our  thoughts  of 
glorification  for  those  architects,  engineers,  physicists, 
chemists,  and  mathematicians  who  have  gone  before,  who 
have  contributed  their  bits  to  improve  their  arts  and  sciences, 
step  by  step,  age  after  age,  until  now  it  has  become  possible 
to  erect  a  Woolworth  building;  an  impossibility  a  few  short 
decades  ago. 

The  completed  edifice  can  not  be  set  down  at  once  for  the 
world  to  admire.  It  must  be  built  up  patiently,  stone  by  stone, 
and  must  rest  upon  a  solid  foundation.  It  is  with  this  idea  in 
mind  that  I  address  you  this  evening.  In  the  first  series  of  the 
Adolfo  Stahl  Lectures  you  were  presented  with  some  aspects 
of  astronomy  of  the  present  time,  the  finished  edifice,  so  to 
speak  (although  no  astronomical  work  is  ever  finished).  In 
this,  the  first  of  the  second  series  of  the  Adolfo  Stahl  Lectures, 
an  attempt  will  be  made  to  show  you  the  various  stepping 

1  Delivered    November    16,    1917. 


128 


THE  ADOLFO  STAHL  LECTURES 


stones  that  have  been  laid  by  the  past  masters  of  the  science 
by  which  alone  it  has  been  possible  to  climb  to  the  great 
heights  attained  by  the  astronomers  of  today. 

In  the  brief  time  allotted  for  a  single  lecture  it  is  impossible 
to  tell  the  whole  story,  so  I  shall  confine  my  remarks  to  the 
most  important  epochs  in  the  development  of  astronomy,  the 
oldest  of  the  sciences. 

In  the  earliest  times  the  notions  concerning  the  form  of  the 
Earth  were  as  numerous  and  varied  as  were  the  peoples. 
About  the  6th  century  B.C.  Pythagoras  and  his  followers 
taught  that  the  Earth  was  spherical  and  a  first  advance  seems 
to  have  been  made.  Many  people  have  the  erroneous  idea  that 
it  was  not  known  that  the  Earth  is  spherical  until  Magellan 
proved  it  by  circumnavigating  the  globe. 

The  first  determination  of  the  distance  from  the  Earth  to 
the  Sun  was  made  by  Aristarchus,  3d  century  B.C.  This 
determination  was  highly  ingenious,  correct  geometrically  but 
yielding  a  very  inaccurate  result.  This  problem  is  so  difficult, 
however,  that  no  good  determination  was  made  until  the  time 
of  Cassini  in  the  17th  century;  so,  great  credit  is  due  Ari- 
starchus for  any  determination  at  this  early  date. 


FIG.  9.     ARISTARCHUS'S  METHOD  OF  DETERMINING  THE  DISTANCE  FROM 
THE  EARTH  TO  THE  SUN. 

To  Eratosthenes,  2d  century  B.C.,  is  due  the  first  measure- 
ment of  the  size  of  the  Earth.  At  noonday,  at  the  summer 
solstice,  the  sun  shone  vertically  down  a  well  at  Syene,  in 
Upper.  Egypt,  while  in  Alexandria,  at  the  same  time,  the 


FIG.  1 — The  Well  of  Eratosthenes. 


FIG.  2 — Newton's  Reflector. 
PLATE  XXXI. 


IMPORTANT  EPOCHS  IN  ASTRONOMY  129 

angular  distance  of  the  Sun  from  the  zenith  was  found  to  be 
approximately  %oth  of  a  complete  circumference,  or  about  7°. 
If  Syene  is  assumed  to  be  directly  south  of  Alexandria,  then 
it  follows,  from  this  observation,  that  the  distance  between 
them  is  %oth  of  the  circumference  of  the  Earth. 

The  geometrical  principle  involved  will  be  clear  from  Fig. 
9.  Since  the  distance  to  the  Sun  is  very  great  compared  to  the 
distance  from  Alexandria  to  Syene,  the  lines  from  these  sta- 
tions to  the  Sun  are  practically  parallel.  Therefore  the  angle 
at  the  center  of  the  Earth  between  radii  drawn  to  the  two  sta- 
tions is  equal  to  the  angle  Z  which  Eratosthenes  measured. 
Hence  the  arc  AS  is  to  the  circumference  of  the  Earth  as  7° 
is  to  360°.  Eratosthenes'  method  is  practically  that  used  today. 
Our  determinations  are  better  only  because  we  can  make  more 
accurate  observations  and  measurements  than  he  could. 

The  greatest  astronomer  of  antiquity  was  undoubtedly 
Hipparchus,  sometimes  called  the  "Father  of  Astronomy,"  who 
flourished  about  the  middle  of  the  2d  century  B.C.  To  him 
we  credit,  among  other  things,  the  invention  of  trigonometry, 
the  first  star  catalogue  of  reasonable  accuracy,  the  discovery  of 
the  precession  of  the  equinoxes,  and,  above  all  else,  the  intro- 
duction of  the  truly  scientific  method  of  observation  and 
investigation. 

The  first  epoch  in  the  development  of  astronomy  may  tbe 
said  to  have  been  brought  to  a  close  upon  the  appearance  of 
Ptolemy's  Almagest,  the  first  great  astronomical  classic. 
Ptolemy  lived  in  the  2d  century  A.D.  He  does  not  seem  to 
have  contributed  much  original  work  to  the  science,  but  he 
rendered  an  inestimable  service  by  bringing  together  in  his 
monumental  work,  and  thus  preserving  for  us,  nearly  all  that 
had  been  done  by  those  who  had  preceded  him. 

The  Almagest  is  noted  principally  for  Ptolemy's  exposition 
of  the  geocentric  theory  of  the  solar  system,  the  theory  that 
the  Earth  is  the  center  of  the  system  and  that  all  of  the  other 
bodies,  including  the  Sun,  revolve  about  it  in  a  system  of 
circles  or  epicycles.  He  sets  forth  the  arguments  pro  and  con 
in  the  controversy  between  the  geocentric  and  the  heliocentric 
theories,  and  then,  perhaps  influenced  by  the  great  weight  of 
Aristotle's  opinion,  rejects  the  heliocentric  theory  and  adopts 
the  geocentric. 


130  THE  ADOLFO  STAHL  LECTURES 

Following  the  time  of  Ptolemy  learning  seems  to  have  gone 
into  nearly  total  eclipse  during  that  period  of  about  fourteen 
centuries  known  as  the  Dark  Ages.  Many  reasons  have  been 
assigned  for  the  decline  of  learning  in  this  period,  but  one  of 
the  principal  causes  is  often  overlooked.  This  is  the  powerful 
influence  of  the  authority  of  Aristotle.  Great  as  Aristotle  may 
have  been  as  a  philosopher,  as  a  scientist  he  was  a  failure. 
This  is  due  to  the  fact  that  the  tenets  he  set  forth  rested  wholly 
upon  thought  analysis  and  were  untested  by  actual  experiment, 
a  wholly  unscientific  method  of  procedure. 

During  this  period  it  was  considered  that  Aristotle  had  done 
all  of  the  world's  thinking,  so  that  there  was  no  further  need 
of  thinking.  His  influence  was  so  great  that  no  one  dared  to 
question  any  of  his  dicta.  If  one,  perchance,  was  so  hardy  as 
to  attempt  it  he  was  probably  immediately  silenced  by  some 
caustic  question  such  as  "Do  you  think  you  know  more  than 
Aristotle  ?"  No  one  seems  to  have  dared  to  think.  No  wonder 
the  people  were  in  the  Dark  Ages !  It  has  been  well  said  that 
Aristotle  did  more  to  retard  the  progress  of  the  world,  at  least 
of  the  scientific  world,  than  any  other  one  man.  How  the 
Dark  Ages  came  to  an  end  will  be  related  presently.  In  the 
meantime  we  come  to  the  next  important  epoch  in  the  develop- 
ment of  astronomy. 

At  the  end  of  the  15th  century  came  Copernicus,  who  gave 
us  the  second  great  astronomical  classic,  the  De  Revolutionibus 
Orbium  Celestium.  In  this  Copernicus  discussed  the  pros  and 
cons  of  the  two  theories  of  the  arrangement  of  the  solar  system 
as  Ptolemy  did,  but  came  to  a  different  conclusion.  Copernicus 
held  the  heliocentric  theory  to  be  the  true  one,  that  is,  that  the 
Sun  is  the  center  of  the  solar  system  and  that  all  of  the  planets, 
of  which  the  Earth  is  one,  revolve  about  the  Sun. 

As  can  readily  be  imagined  this  doctrine  did  not  find  ready 
acceptance.  For  many  centuries  the  old  geocentric  idea  had 
held  sway  and  it  was  not  to  be  overthrown  easily.  It  was  about 
half  a  century  later  that  the  observations  and  teachings  of 
Galileo  gave  the  final  push  to  the  tottering  geocentric  theory 
and  it  fell. 

From  the  middle  of  the  16th  century  to  the  middle  of  the 
17th  we  find  three  epoch-making  names  in  astronomy,  Tycho 


IMPORTANT  EPOCHS  IN  ASTRONOMY  131 

Brahe,  Kepler,  and  Galileo.  The  latter  two  were  contempo- 
raries following  immediately  after  Tycho. 

Beginning  at  this  time  we  find  a  revival  in  observational 
work.  This  was  carried  on  partly  in  Cassel,  where  we  find  the 
first  observatory  built  with  a  revolving  dome  and  the  first  use 
of  a  clock  to  record  time  observations.  But  the  principal 
observational  work  then  was  done  by  Tycho  at  Hveen.  He 
recognized  the  need  of  more  accurate  positions  of  the  Sun, 
Moon,  planets  and  stars  than  were  available.  Tycho,  therefore, 
erected  an  elaborate  observatory  and  equipped  it  with  as  fine 
instruments  as  were  then  possible.  While  he  made  no  startling 
discovery,  he  amassed  a  store  of  very  accurate  observations, 
especially  of  the  planets,  which  were  destined  soon  to  play  a 
very  important  role  in  the  development  of  astronomy. 

Tycho's  valuable  observations  fortunately  fell  into  the  hands 
of  Kepler,  who  mined  through  them  with  remarkable  patience 
and  perseverance.  Up  to  this  time  the  positions  of  the  planets 
had  been  predicted  on  the  assumption  that  they  moved  in  circles 
or  combinations  of  circles.  But  Kepler  was  soon  able  to  show 
from  Tycho's  accurate  observations  that  they  could  not  move 
in  such  a  way.  He  then  set  about  to  discover  the  true  paths. 
After  much  trying  and  guessing  he  at  last  found  that  their 
motion  could  be  represented  accurately  by  ascribing  the  ellipse 
as  the  true  path,  with  the  Sun  situated  at  one  focus  of  the 
ellipse.  We  have  here  the  first  departure  from  the  old  idea 
of  motion  in  a  circle. 

Then  Kepler  reasoned,  that  as  a  planet  is  at  varying 
distances  from  the  Sun  on  account  of  moving  in  an  ellipse,  it 
probably  would  move  with  a  variable  velocity.  Again  he 
started  his  trying  and  guessing  and  finally  hit  upon  his  second 
wonderful  law,  namely,  that  a  planet  moves  in  such  a  way  that 
the  line  joining  the  Sun  and  the  planet  describes  equal  areas  in 
equal  intervals  of  time. 

Once  more  his  active  brain  got  to  work  and  he  began  to 
consider  the  fact  that  as  the  various  planets  are  at  different 
distances  from  the  Sun  there  is  probably  some  relation  between 
the  distances  and  the  time  of  revolution  of  the  various  planets 
about  the  Sun.  If  one  planet  is  twice  as  far  from  the  Sun  as 
another,  will  it  take  twice  as  long  to  go  once  completely  around 


132  THE  ADOLFO  STAHL  LECTURES 

the  Sun  ?  He  soon  saw  that  no  such  simple  relation  held  true. 
So  again  he  started  his  trying  and  guessing  and  finally  arrived 
at  his  wonderful  Harmonic  Law,  which  states  that  the  squares 
of  the  periods  (expressed  in  years)  are  equal  to  the  cubes  of 
the  mean  distances  from  the  Sun  (the  Earth's  distance  being 
taken  as  unity). 

You  have  probably  gathered  from  all  this  that  Kepler  was 
the  world's  champion  guesser,  and  he  probably  was.  Even  so, 
he  could  not  have  arrived  at  these  three  wonderful  laws  had 
he  not  had  his  material  systematically  arranged.  We  are  told 
that  he  chose  his  second  wife  after  investigations  most  method- 
ical. He  put  down  on  cards  the  points  of  merit  of  each  one  of 
nearly  a  dozen  maidens,  and  after  studying  them  carefully, 
made  his  choice.  Kepler  was  probably  the  founder  of  our 
present  elaborate  card  index  system. 

This  epoch  marked  by  Kepler  and  Galileo  is  certainly  a 
noted  one  in  the  development  of  astronomy.  While  Kepler 
was  discovering  his  beautiful  laws  of  planetary  motion, 
Galileo  in  Italy  was  doing  work  which  in  itself  was  epochal. 
When  he  applied  the  telescope  to  the  skies  for  the  first  time  a 
vast  new  field  of  investigation  was  started.  This  is  so  evident 
that  we  shall  not  dwell  upon  it  further.  In  addition  to  this, 
however,  he  made  several  other  contributions  to  science  which 
would  have  made  him  famous,  an  account  of  which  would 
require  several  lectures.  We  shall  dwell  upon  only  two.  One 
of  these  is  his  support  of  the  Copernican  heliocentric  theory, 
which  he  upheld  in  his  famous  work  entitled  The  Dialogue  of 
the  Tivo  Systems.  It  was  this  support  of  Galileo  that  un- 
doubtedly hastened  its  general  acceptance. 

I  wish  to  dwell  upon  the  second  of  these  contributions  more 
at  length,  as  it  marks  one  of  the  greatest  epochs  in  the  develop- 
ment, not  only  of  astronomy,  but  in  the  whole  realm  of  thought. 
It  concerns  that  movement  at  the  end  of  the  Dark  Ages  known 
as  the  Revival  of  Learning,  or  Renaissance.  This  probably 
began  in  the  15th  century,  but,  in  my  opinion,  the  rapid  revival 
(so  rapid,  indeed,  as  to  make  it  practically  the  Revival  itself) 
began  with  Galileo. 

As  mentioned  before,  the  dicta  of  Aristotle  had  held  sway 
over  the  world's  thought  for  some  fifteen  centuries.  Among 


PLATE  XXXII.     SIR  ISAAC  NEWTON,  1642-1727. 


IMPORTANT  EPOCHS  IN  ASTRONOMY  133 

other  things  he  had  said  that  a  large  heavy  body  would  fall 
faster  than  a  small  light  body.  Galileo,  true  scientist  as  he  was, 
would  not  take  the  word  of  anyone,  even  Aristotle,  for  a  thing 
when  it  was  in  his  power  to  prove  or  disprove  the  statement. 
So,  mounting  to  the  top  of  the  Leaning  Tower  of  Pisa,  before  a 
large  gathering  of  interested  people,  he  let  fall  from  this  height 
simultaneously  two  bodies,  one  large  and  heavy,  the  other  small 
and  light.  According  to  Aristotle  the  heavy  body  ought  to  have 
reached  the  ground  much  sooner  than  the  light  one.  But  the 
astonished  people  saw  the  two  falling  side  by  side,  and  landing 
at  the  base  of  the  tower  at  practically  the  same  instant.  This 
was  one  of  the  most  dramatic  experiments  the  world  has 
known.  With  the  fall  of  those  bodies  fell  the  influence  of 
Aristotle  in  matters  scientific.  You  can  easily  imagine  that, 
with  the  disproving  of  one  of  his  dicta,  the  others  immediately 
came  into  question.  Investigators  in  all  lines  of  thought  soon 
began  to  appear,  and  the  rapid  revival  of  learning  was  on. 

Galileo  died  in  1642,  and  Newton  was  born  in  1643.  The 
interval  between  Galileo's  death  and  the  epoch  marked  by 
Newton's  activities  was  one  of  steady  progress.  Instruments 
were  improved  somewhat,  and  came  into  more  general  use; 
the  micrometer  was  invented ;  accurate  measurements  of  lengths 
of  arcs  in  different  latitudes  were  made  from  which  it  was 
found  that  the  Earth  is  not  a  perfect  sphere,  but  an  oblate 
spheroid.  Observational  work  was  carried  on  most  assiduously, 
especially  at  the  Paris  Observatory.  In  addition  to  the  dis- 
covery of  the  true  form  of  the  Earth,  the  most  important 
developments  at  this  time  were  the  discovery  of  the  finite 
velocity  of  light  by  Romer  working  under  Cassini  at  the  Paris 
Observatory,  and  Cassini's  evaluation  of  the  distance  from  the 
Sun  to  the  Earth.  He  deduced  the  value  9.5"  for  the  parallax 
of  the  Sun,  which  corresponds  to  a  distance  of  78,000,000  miles. 
This  is  the  first  fairly  good  approximation  to  that  all-important 
distance. 

The  next  great  epoch  in  the  development  of  astronomy  is 
that  connected  with  the  name  of  Sir  Isaac  Newton,  who  lived 
from  1643  to  1727.  The  work  of  this  man,  whose  name  is 
undoubtedly  the  greatest  in  astronomy,  is  so  wonderful  both  in 
quantity  and  quality  that  it  is  difficult  to  know  what  to  say 
about  it  in  the  few  minutes  available. 


134  THE  ADOLFO  STAHL  LECTURES 

The  name  of  Newton  suggests  immediately  his  law  of 
Universal  Gravitation,  viz. :  that  every  particle  in  the  universe 
attracts  every  other  particle  in  the  universe  with  a  force  that 
is  proportional  to  the  product  of  the  masses  of  the  two  particles 
and  inversely  proportional  to  the  square  of  the  distance  be- 
tween them.  It  would  unfortunately  take  too  much  time  to  tell 
how  he  came  to  arrive  at  this  law.  This  is  the  law  upon  which 
all  work  in  theoretical  astronomy  and  celestial  mechanics  is 
based.  Newton  is  properly  called  the  "Father  of  Gravitational 
Astronomy". 

Kepler  had  shown  in  his  three  laws  the  manner  in  which  the 
planets  move,  but  he  had  not  the  slightest  idea  of  why  they 
move  in  this  way.  For  some  time  a  vague  idea  was  prevalent 
that  there  was  some  such  thing  as  a  gravitational  force  residing 
in  the  Sun,  but  it  remained  for  Newton  to  formulate  it  and 
prove  it.  Starting  with  his  law  Newton,  'with  his  mathematical 
genius,  was  able  to  prove  Kepler's  Laws  and  to  show  that,  act- 
ing under  the  gravitational  influence  of  the  Sun  in  this  way, 
they  must  move  as  described  by  Kepler.  To  do  this  and  to 
prove  other  problems  in  theoretical  astronomy  the  necessary 
mathematics  were  not  available  in  Newton's  time,  so  he  had 
first  of  all  to  bring  his  marvelous  mathematical  talents  into 
play.  As  a  result  of  this  he  invented  the  all-powerful  mathe- 
matical tool  known  as  calculus.2 

With  these  and  other  mathematical  tools  which  his  sagacity 
gaye  us  he  was  now  able  to  explain  many  things  which  were 
awaiting  explanation,  among  which  may  be  mentioned  the 
precession  of  the  equinoxes,  discovered  by  Hipparchus  nearly 
twenty  centuries  previously;  certain  inequalities  in  the  motion 
of  the  Moon;  perturbations  in  general,  and  the  tides.  Another 
field  of  investigation  was  thus  opened  up  which  was  quite  as 
vast  as  that  started  by  Galileo. 

In  addition  to  founding  gravitational  astronomy  Newton 
may  also  be  credited  with  the  founding  of  modern  astronomy 
or  astrophysics  when  he  discovered  the  composite  character  of 
white  light,  the  phenomenon  by  which  white  light,  when  passed 
through  a  prism,  is  broken  up  into  its  constituent  parts  and 

2  The  honor  of  inventing  calculus  has  to  be  shared  perhaps  with  the  German 
mathematician,  Leibnitz. 


PLATE  XXXIII.    SIR  WILLIAM  HERSCHEL,  1738-1822. 


IMPORTANT  EPOCHS  IN  ASTRONOMY  135 

spread  out  into  a  band  of  color  which  we  call  the  spectrum. 
Although  spectrum  analysis  did  not  begin  to  develop  until  the 
middle  of  the  19th  century,  its  foundation  was  laid  by  Newton. 

With  this  discovery  of  the  composite  character  of  light 
Newton  was  enabled  to  give  the  correct  explanation  of  the 
phenomenon  known  as  chromatic  aberration,  that  trouble  which 
the  astronomers  of  that  time  were  having  with  the  fringe  of 
color  around  an  image  which  affected  the  sharpness  of  the 
image.  Here  Newton  made  a  mistake  in  his  scientific  work,  for 
he  decided  that  this  trouble  could  not  be  overcome.  We  now 
know  how  to  eliminate  chromatic  aberration  almost  completely 
by  making  the  object  glass  not  of  a  single  lens,  but  of  two  or 
more  lenses  of  different  density.  But  this  discovery  was  made 
half  a  century  after  the  time  of  Newton.  We  should  be  very 
thankful,  however,  that  he  made  this  mistake,  for  it  led  him 
to  the  invention  of  the  reflecting  telescope.  Whereas  the 
amount  of  bending  of  a  ray  of  light  upon  passing  through  a 
prism  or  lens  depends  upon  its  color,  all  rays,  no  matter  what 
the  color,  obey  the  same  law  of  reflection,  viz. :  that  the  angle  of 
incidence  is  equal  to  the  angle  of  reflection.  Newton,  therefore, 
saw  that  if  the  rays  were  brought  to  a  focus  on  the  principle  of 
reflection,  the  image  would  be  freed  from  chromatic  aberration, 
as  all  of  the  rays,  regardless  of  color,  would  be  brought  to  the 
same  focus,  and  a  white  object  would  yield  a  white  image  with 
no  halo  of  colors  to  bother.  Acting  upon  this  idea  he  con- 
structed the  first  reflecting  telescope,  a  modest  affair  with  a 
one-inch  mirror  made  of  a  combination  of  copper  and  tin  and 
highly  polished.  This  instrument  of  such  historical  interest  is 
still  to  be  seen  as  one  of  the  priceless  exhibits  in  the  library  of 
the  Royal  Society  in  London.  Although  small  and  inefficient 
this  little  instrument  is  the  forerunner  of  the  increasingly  large 
and  valuable  reflectors  culminating  in  our  own  time  in  the 
72-inch  reflector  of  the  Dominion  Astrophysical  Observatory 
and  the  monster  100-inch  reflector  about  to  be  put  into  opera- 
tion at  the  Mount  Wilson  Solar  Observatory. 

The  results  of  most  of  the  work  of  Sir  Isaac  Newton  were 
given  to  the  world  in  the  publication  of  his  Principia,  issued  in 
three  large  volumes. 

Beginning  with  the  time  of  Newton  we  find  astronomy 


136  THE  ADOLFO  STAHL  LECTURES 

developing  so  rapidly  that  its  progress  must  now  be  traced,  not 
as  a  whole,  but  along  its  various  branches. 

Just  before  the  time  of  Newton  the  principal  observational 
work  had  been  done  by  the  continental  astronomers.  One 
would  naturally  expect  the  next  advances  in  the  observational 
line  to  be  developed  by  them  and  the  theoretical  branch  to  be 
explored  by  the  English,  following  Newton.  But,  curiously, 
the  converse  was  the  case. 

During  the  18th  century  the  principal  observational 
advances  were  made  by  the  English.  Toward  the  close  of  the 
17th  century  Flamsteed,  the  first  Astronomer  Royal,  founded 
the  observatory  at  Greenwich.  On  account  of  the  lack  of  funds 
his  instrumental  equipment  was  very  meager  and  he  did  little 
beyond  making  a  star  catalogue.  The  next  two  Astronomers 
Royal,  Halley  and  Bradley,  however,  did  much  to  advance  the 
science. 

Halley  was  a  great  admirer  of  Newton.  Following  some  of 
the  lines  of  Newton's  work  he  computed  the  paths  of  some 
twenty-four  comets.  Noting  that  three  of  these  were  traveling 
in  practically  the  same  path,  separated  from  each  other  at 
nearly  equal  intervals  of  75  or  76  years,  Halley  ventured  the 
idea  that  these  were  not  three  separate  comets  but  were  three 
appearances  of  one  and  the  same  comet  at  about  75-year  inter- 
vals. He  predicted  the  return  of  the  comet.  He  did  not  live 
until  the  predicted  year,  but  the  comet  appeared  on  schedule. 
This  is  the  first  instance  of  the  kind  in  history,  and  the  comet 
is  named,  as  you  know,  Halley's  comet,  in  honor  of  the  man 
who  first  predicted  its  return.  The  return  which  he  announced 
took  place  in  1759.  It  has  returned  again  twice  since  then,  in 
1835  and  in  1910.  This  marked  another  advance  in  astronomy 
in  that  it  added  other  moving  bodies  to  the  solar  system,  and 
showed  the  people  that  comets  were  no  longer  to  be  feared, 
that  they  were  merely  members  of  the  solar  system,  moving 
under  the  attraction  of  the  Sun,  and  that  their  motions  and 
positions  could  be  computed  in  accordance  with  Newton's  and 
Kepler's  Laws  just  the  same  as  in  the  case  of  the  planets. 

Among  other  things  done  by  Halley  of  a  largely  developing 
character  may  be  mentioned  his  discovery  of  the  proper  motions 
of  the  stars ;  and  his  scheme  for  determining  the  solar  parallax. 


PLATE  XXXIV.    SIR  WILLIAM  HUGGINS,  1824-1910. 


IMPORTANT  EPOCHS  IN  ASTRONOMY  137 

and  hence  the  distance  from  the  Sun  to  the  Earth,  by  observa- 
tions of  the  transit  of  Venus. 

Bradley,  the  successor  Of  Halley,  discovered  aberration  and 
nutation.  All  of  the  objects  in  the  sky  are  displaced  somewhat, 
due  to  the  facts  that  the  Earth  is  moving  and  that  the  velocity 
of  light  is  not  infinite.  This  displacement  is  known  as  aberra- 
tion. It  was  discovered  by  Bradley  quite  by  accident  in  seeking 
to  find  a  displacement  of  the  stars  due  to  parallax. 

We  must  credit  Bradley  also  with  improving  the  accuracy 
of  observational  work.  Not  only  was  he  a  keen  observer,  but 
he  had  his  instruments  constructed  by  the  most  skillful 
mechanics  and  mounted  in  the  best  possible  manner.  Further, 
he  was  one  of  the  first  to  take  into  account  possible  errors 
arising  from  defects  of  his  instruments. 

Contrary  to  natural  expectation  we  find  but  little  done  in 
the  18th  century  in  England  along  the  lines  of  Gravitational 
Astronomy.  This  country  does  not  seem  to  have  possessed 
minds  of  a  caliber  to  follow  the  lead  of  the  immortal  Newton. 
His  work  was  principally  geometrical  and  for  this  reason 
difficult  to  master.  On  the  continent,  however,  there  appeared 
a  group  of  mathematical  astronomers  who  founded  and 
developed  what  is  called  the  method  of  analysis,  that  is,  a 
development  following  the  lines  of  algebra.  With  this  they 
were  enabled  to  develop  the  planetary  and  lunar  theories  of 
celestial  mechanics  to  a  very  high  state.  The  leaders  in  this 
work  were  Euler,  a  Swiss,  Clairaut,  D'Alembert,  Lagrange, 
and  Laplace  of  the  French  school.  Lack  of  time  prevents  a 
detailed  statement  of  what  they  accomplished.  A  compre- 
hensive discussion  of  the  state  of  the  developments  at  this  time 
is  published  in  the  monumental  work  of  Laplace  entitled  the 
Mecanique  Celeste.  Among  the  interesting  and  important 
things  done  by  Laplace  may  be  mentioned  his  proof  of  the 
stability  of  the  solar  system,  and  of  the  fact  that  Saturn's  rings 
could  not  be  solid.  The  physical  proof  of  the  latter  was  not 
accomplished  until  near  the  end  of  the  19th  century,  when  it 
was  beautifully  demonstrated  by  Keeler. 

Laplace  also  set  forth  in  a  beautiful  work,  the  Systcme  du 
Monde,  the  Nebular  Hypothesis  that  goes  under  his  name. 
This  theory  of  the  formation  of  the  solar  system  held  sway  for 
more  than  a  century;  and  while  we  are  now  obliged  to  reject 


138  THE  ADOLFO  STAHL  LECTURES 

it  in  the  precise  formulation  given  by  Laplace,  the  funda- 
mental idea  in  it,  that  of  the  evolution  of  the  solar  system  from 
a  primal  nebula,  is  still  accepted  by  astronomers. 

During  the  latter  part  of  the  18th  century  and  the  first  part 
of  the  19th  observational  astronomy  was  carried  to  great 
heights  by  Sir  William  Herschel,  working  with  his  devoted 
sister  Caroline  at  Slough,  England.  His  remarkable  work  was 
made  possible  by  the  large  instruments,  reflectors,  that  he  made, 
culminating  in  his  40- foot  telescope. 

He  added  the  knowledge  of  the  existence  of  the  planet 
Uranus  by  discovering  it  accidentally  in  1781.  He  made  many 
other  observational  discoveries,  but  his  most  important  develop- 
ment work  was  his  discovery  of  binary  star  systems  and  his 
general  survey  of  the  sky  for  nebulae  and  the  distribution  of  the 
stars.  These  led  him  to  speculate  on  the  form  of  the  sidereal 
universe  and  mark  the  beginning  of  that  wonderful  wqrk  that 
is  the  principal  problem  of  the  20th  century  astronomers.  His 
investigations  led  him  to  set  forth  his  so-called  "grindstone 
theory,"  that  is,  that  the  universe  is  shaped  like  a  grindstone, 
having  the  greatest  depth  in  the  plane  of  the  Milky  Way. 

The  developments  in  the  19th  century  were  so  numerous  as 
to  become  almost  bewildering  when  one  tries  to  narrate  them. 
We  can  at  best  mention  here  but  few.  Gravitational  astronomy 
has  been  improved  and  developed  along  the  lines  laid  down  by 
the  five  famous  mathematicians  of  the  18th  century,  in  large 
part  by  Pontecoulant,  Delaunay,  Leverrier,  Poincare,  Gauss, 
Hansen,  Gylden,  Adams,  and  our  own  Newcomb  and  Hill. 
Time  does  not  permit  of  the  enumeration  of  the  individual 
contributions  of  these  and  yet  others.  But  I  will  pause  to 
mention  two  things.  The  first  is  the  discovery  of  Neptune  in 
1846,  which  resulted  from  the  computational  work  of  Adams, 
of  England,  and  Leverrier,  of  France.  The  other  is  that  the 
best  lunar  theory  we  have  is  that  given  us  by  the  famous 
American,  G.  W.  Hill,  to  whom  the  Bruce  Medal  of  the  Astro- 
nomical Society  of  the  Pacific  was  awarded  shortly  before  his 
death  a  few  years  ago. 

Observational  astronomy  progressed  with  rapid  strides  in 
the  last  century,  due  to  the  improvements  in  the  size,  number, 
and  quality  of  instruments,  and  in  the  mathematical  methods 
deduced  for  handling  observational  material.  The  latter  are 


PLATE  XXXV.    SIMON  NEWCOMB,  1835-1909. 


IMPORTANT  EPOCHS  IN  ASTRONOMY  139 

due  principally  to  Gauss  and  Bessel.  We  owe  to  Bessel  also 
the  first  detection  of  the  parallax  of  a  star.  Instruments  were 
made  much  larger  and  better,  principally  here  in  America, 
culminating  toward  the  end  of  the  century  in  the  great 
refractors  of  the  Lick  and  the  Yerkes  Observatories. 

Toward  the  end  of  the  century  two  important  discoveries 
were  made  which  must  be  mentioned.  The  first  is  the  discovery 
of  the  variation  of  latitude  by  Kiistner  at  Bonn;  the  other  is 
the  discovery  made  by  Keeler  at  the  Lick  Observatory  that  the 
spiral  nebula  is  the  rule  and  not  the  exception  among  the 
nebulae.  The  latter  gave  the  final  blow  to  the  nebular  hypothe- 
sis as  formulated  by  Laplace  and  started  astronomers  again  to 
speculating  upon  the  structure  of  the  universe. 

Finally,  in  this  hurried  review,  we  come  to  note  the  develop- 
ment of  modern  astronomy  or  astrophysics.  This  was  started, 
as  was  related,  when  Sir  Isaac  Newton  discovered  the  composite 
character  of  light.  Early  in  the  19th  century  Fraunhofer  had 
noted  and  mapped  certain  dark  lines  running  across  the 
spectrum  of  the  Sun,  the  lines  which  are  named  after  him.  The 
true  explanation  of  these  was  given  about  the  middle  of  the 
century  by  Kirchhoff,  and  the  new  science  of  astrophysics  was 
well  started.  Here  again  was  opened  up  a  new  field  of  investi- 
gation quite  as  large  as  that  started  by  Galileo  with  his 
telescope. 

In  this,  the  20th  century,  the  reflecting  telescope  is  out- 
stripping the  refractor.  We  have  ever  larger  and  larger 
reflectors,  culminating,  as  has  been  told,  in  the  100-inch  reflector 
at  Mount  Wilson.  Hand  in  hand  with  this  goes  the  develop- 
ment of  spectroscopes  and  minor  apparatus,  and  the  perfection 
of  photographic  processes.  Keeler's  discovery  concerning  the 
nebular  forms,  and  the  development  of  astrophysics  mark  an 
epoch  which  starts  the  20th  century  well  on  its  way  to  master 
the  great  unsolved  problem  of  astronomy,  the  structure  of  the 
universe. 

I  am  only  too  well  aware  of  how  inadequate  this  sketch  is ; 
many  more  things  perhaps  should  have  been  said.  I  hope, 
however,  that,  incomplete  as.  it  is,  this  account  has  given  you 
some  idea  of  the  various  foundation  stones  upon  which  the 
beautiful  astronomical  superstructure  has  been  erected. 


OUR  NEAREST  STAR,  THE  SUN1 

By  CHARLES  E.  ST.  JOHN 

If  the  Sun  were  removed  to  eight  times  the  distance  of  its 
nearest  stellar  neighbor,  it  would  appear  among  the  fainter 
stars,  just  fairly  visible  to  the  unaided  eye.  Like  the  other 
stars,  it  is  self-luminous,  but  among  them  it  is  conspicuous  only 
because  of  its  relative  nearness,  as  there  are  many  other  stars 
that  surpass  it  in  size  and  greatly  excel  it  in  luminosity.  The 
blazing  Sirius,  the  brightest  star  in  all  the  sky,  has  3.4  times 
the  mass  of  the  Sun  and  sends  out  48  times  the  light,  but  even 
it  is  far  surpassed  in  absolute  luminosity  by  other  giant  stars. 

Nevertheless  the  importance  of  the  Sun  to  us  is  typified 
by  its  apparent  prominence  in  the  heavens,  for,  in  a  very  real 
sense,  we  are  children  of  the  Sun.  The  Earth  is  held  in  her 
path  by  the  invisible  attraction  of  the  Sun,  a  pull  greater  than 
could  be  exerted  by  a  bond  of  steel  hundreds  of  miles  in 
diameter ;  or,  as  Young  puts  it,  it  would  be  necessary  to  cover 
the  whole  Earth  with  wires  as  large  as  telegraph  wires  and 
only  about  half  an  inch  apart  in  order  to  get  a  metallic  connec- 
tion that  would  stand  the  strain.  Not  only  is  the  motion  of 
the  Earth  in  space  controlled  by  the  masterful  Sun,  but  what 
is  more  directly  evident,  the  Sun  is  the  source  of  practically 
all  our  light  and  heat,  without  which  life,  as  we  know  it,  could 
not  exist  upon  the  Earth.  Some  one  has  said  that  if  the  Earth 
were  cut  off  from  all  solar  radiation  for  a  single  month,  all  life 
would  be  extinguished  and  the  world  become  a  frozen  waste. 

It  is  not  so  evident,  but  as  clearly  true,  that  the  energy 
stored  in  wood,  coal,  oil  and  gas  has  come  to  us  from  the  Sun. 
Under  the  influence  of  sunlight,  particularly  of  the  red  and 
blue  components,  the  carbon  dioxide  of  the  atmosphere  is 
taken  in  by  the  leaves  of  trees  and  plants  and  acted  upon  to 
form  the  complex  constituents  of  plant  growth,  mainly  com- 
pounds of  carbon  with  hydrogen,  oxygen  and  nitrogen.  Their 
chemical  transformation  requires  the  absorption  of  energy 


1  Delivered  December   14,    1918. 


OUR  NEAREST  STAR,  THE  SUN  141 

which  is  accumulated  and  stored  in  these  compounds,  to  be 
released  and  again  transformed  when  they  are  burned  rapidly 
in  ordinary  combustion,  or  slowly  in  our  own  bodies.  Every 
heart  beat,  every  breath  we  take,  every  thought,  and  every  act 
performed  draws  its  working  power  from  the  accumulated 
solar  energy  stored  up  in  plant  and  animal  growth.  The  trans- 
formation of  solar  energy  in  plant  growth  takes  place  in  the 
leaves  under  the  action  of  sunlight  upon  the  green  coloring 
matter,  the  chlorophyll.  As  heat  engines  plants  cannot  be  con- 
sidered efficient,  transforming  as  they  do  only  one  or  two  per 
cent  of  the  solar  energy  falling  upon  their  leaves,  but  the 
energy  supplied,  as  will  appear  later,  is  enormous ;  plants  work 
continually  during  growth  and  store  up  energy  in  permanent 
form;  these  are  favorable  conditions  and  result  in  tremendous 
advantages  for  man.  The  energy  of  coal  has  waited  for  his 
touch  many  millions  of  years  and  what,  if  any,  escapes  his 
wasteful  use  will  endure  uncounted  millions  yet  without  loss 
of  its  potential  energy.  The  energy  of  the  Sun  is  stored  in 
the  water  lifted  into  the  atmosphere  by  the  Sun's  power  and 
carried  by  wind-driven  clouds  to  higher  regions,  whence  it 
falls  as  rain  or  snow,  ever  renewing  the  reservoirs  and  so 
rendering  them  a  practically  exhaustless  source  of  power. 

The  study  of  the  Sun  is  of  interest  not  only  for  its 
immediate  importance  to  us,  but  because  the  Sun  is  the  only 
star  near  enough  to  us  to  allow  of  intensive  and  detailed  study. 
For  a  proper  orientation  it  may  be  well  to  consider  some  of 
the  tremendous  magnitude  relations  of  the  Sun. 

The  diameter  of  the  Sun 863,000  miles 

The  distance  from  the  Earth 93,000,000  miles 

The  mass  of  the  Sun 332,000  X  Earth 

The  mass  of  the  Earth 6.58  X  1019  tons 

The  mass  of  the  Sun 2.19  X  1027  tons 

Distance  to  nearest  star 25  X  1012  miles 

It  is  impossible  for  us  to  conceive  the  meaning  of  such 
colossal  numbers,  but  they  serve  to  indicate  relations ;  and  they 
make  it  less  surprising  that  we  know  so  little  than  amazing  that 
we  have  learned  so  much  concerning  bodies  at  such  inconceiv- 
able distances,  and  that  the  human  mind  has  been  able  to  bridge 


142  THE  ADOLFO  STAHL  LECTURES 

such  vast  spaces  and  bring  to  our  knowledge  more  and  more 
the  secrets  of  the  universe. 

The  advancement  of  our  knowledge  of  the  Sun  and  stars 
depends  in  great  measure  upon  the  analysis  of  light  by  the 
spectroscope,  an  instrument  by  which  the  white  light  of  the 
Sun  is  stretched  out  into  a  spectrum,  that  is,  a  narrow  band  of 
colors  extending  from  red  through  yellow,  green  and  blue,  to 
violet,  crossed  at  right  angles  by  a  vast  number  of  narrow 
dark  lines.  It  is  to  these  dark  lines,  the  Fraunhofer  lines,  that 
the  solar  investigator  gives  his  attention  rather  than  to  the 
brilliantly  colored  band.  From  the  changes  in  the  relative 
positions,  intensities,  and  other  characteristics  of  these  dark 
lines,  he  determines  the  substances  in  the  Sun,  the  pressure 
and  motions  in  the  atmosphere,  the  law  of  its  rotation,  the 
temperature  and  magnetic  effects  in  sun-spots,  and  endeavors 
to  find  answers  to  the  many  as  yet  unsolved  problems.  The 
spectrum  is  to  most  people  a  kind  of  unknown  language.  The 
interpretation  of  its  message  from  the  Sun  and  the  far  more 
distant  stars  is  the  special  work  of  the  astronomer.  He  finds 
the  key  to  it  in  the  physical  laboratory,  which  forms  an 
essential  part  of  a  modern  solar  observatory.  When  in  the 
laboratory  a  substance  like  iron,  for  example,  is  turned  to 
vapor  at  a  very  high  temperature,  the  iron  vapor  becomes 
luminous  and  emits  a  characteristic  light.  This  light  when 
analyzed  by  a  spectroscope  yields,  not  a  band  of  colors,  but  a 
series  of  narrow  bright  lines  scattered  through  the  red,  yellow, 
green,  blue  and  violet.  Each  element  when  in  the  form  of 
vapor  may  be  made  to  yield  a  line  spectrum  which  dis- 
tinguishes it  from  every  other  element  and  furnishes  the  means 
for  its  positive  identification.  Moreover,  incandescent  vapors 
absorb  from  white  light  passing  through  them  precisely  the 
rays  which  they  by  themselves  emit,  so  that  under  suitable 
conditions  of  temperature  and  emission,  the  spectrum  of  the 
transmitted  white  light  shows  dark  (absorption)  lines  in  the 
exact  positions  of  the  bright  lines  that  characterize  the 
spectrum  of  the  vapor,  and  these  dark  lines  serve  equally  well 
for  its  identification. 

These  principles  are  used  in  the  identification  of  substances 
in  the  atmosphere  of  the  Sun  and  stars.  The  vapors  and 


FIG.  1 — Coincidence  of  bright  lines    (a)    from  iron  vapor  with  dark  lines 
(b)  in  the  Sun's  spectrum,  violet  region,  M200. 


FIG.  2 — Displacement  of  the  lines  at  the  east  (c)   and  west   (d)   limbs  of 
the  Sun,  green  region,  X5167. 


FIG.  3 — Lines  of  B  group  due  to  oxygen  in  the  Earth's  atmosphere  undis- 
placed  by  Sun's  rotation,   red  region,  ^.6867. 


FIG.  4 — Displacement  of  the  lines  on  the  near  (e)  and  far  (f)  sides  of  a 
sun-spot  showing  outflow,  blue  region,  X4765. 


PLATE  XXXVI. 


OUR  NEAREST  STAR,  THE  SUN  143 

gases  in  these  atmospheres,  though  at  temperatures  of  thou- 
sands of  degrees,  are  cooler  than  the  source  of  the  white  light 
originating  lower  down  and,  as  this  passes  through  them  on 
its  way  to  us,  it  impresses  upon  its  own  spectrum  their 
characteristic  absorption  lines.  A  portion  of  the  violet  region 
in  the  spectra  of  the  Sun  and  of  the  glowing  vapor  of  iron  is 
reproduced  in  Fig.  1,  Plate  XXXVI.  The  coincidence  be- 
tween the  bright  lines  of  the  iron  spectrum  and  dark  lines  in  the 
Sun's  spectrum  is  complete  and  shows  therefore  the  presence  of 
incandescent  iron  vapor  in  the  solar  atmosphere. 

Of  the  92  elements  indicated  by  the  periodic  system  all 
except  five  or  six  have  been  found  upon  the  Earth,  some  in 
minute  amounts  only.  The  number  of  elements  identified  with 
certainty  in  the  Sun  is  38  and  includes  the  common  metals — 
iron,  nickel,  copper,  zinc,  lead,  tin.  Of  most  of  the  heavy 
metals,  such  as  gold,  platinum,  iridium,  and  uranium,  there  is 
no  positive  evidence.  If  they  are  represented  at  all  in  the  solar 
spectrum  it  is  only  by  the  faintest  lines.  The  absence  of 
definite  evidence  of  the  presence  of  these  heavy  elements  in 
the  Sun  may  be  due  in  part  to  their  actual  rarity.  If  the  92 
possible  chemical  elements  be  arranged  in  the  order  of  increas- 
ing atomic  numbers,  it  is  found,  as  Professor  Harkins  points 
out,  that  the  comparatively  light  elements  occurring  in  the 
first  third  of  the  series  supply  99  per  cent  of  the  substances  in 
the  Earth's  accessible  crust  and  in  meteorites :  i.  e.,  two-thirds 
of  the  elements,  the  heavier  ones,  furnish  only  a  fraction  of 
one  per  cent  of  the  Earth's  crust  and  of  the  cosmic  material 
represented  by  the  meteoric  visitors  from  interstellar  space. 
If  the  proportions  between  the  light  and  heavy  elements  and 
their  distribution  in  the  Sun  are  comparable,  as  seems 
probable,  with  the  proportions  and  distribution  in  terrestrial 
sources,  there  can  be  at  most  only  traces  of  them  in  the  lower 
levels  of  the  solar  .atmosphere  and  it  is  not  surprising  that  we 
have  not  yet  detected  them  with  certainty.  The  groups  of 
non-metallic  elements,  such  as  chlorine  and  bromine,  oxygen 
and  sulphur,  nitrogen  and  phosphorus,  are  not  represented  in 
the  solar  spectrum  by  their  characteristic  lines,  except  possibly 
oxygen  and  nitrogen.  The  suggested  explanation  is  found  in 
the  observation  that  the  presence  of  metallic  vapors  tends  to 


144  THE  ADOLFO  STAHL  LECTURES 

suppress  the  spectra  of  the  non-metals  when  the  two  classes 
of  substances  occur  in  the  same  mixture. 

As  a  locomotive  whistle  is  higher  in  pitch  when  the  train 
is  approaching  than  when  receding  from  the  observer,  so 
light  coming  from  a  rapidly  approaching  source  is  bluer,  and 
from  a  receding  source  is  redder,  than  when  the  source  is  at 
rest;  this  manifests  itself  in  the  spectrum  by  a  slight  displace- 
ment of  the  lines  toward  the  violet  or  toward  the  red  accord- 
ing as  the  source  is  approaching  or  receding.  This  is  known  as 
the  Doppler  effect.  In  Fig.  2,  Plate  XXXVI,  are  shown  nar- 
row spectra  taken  from  the  east  and  west  edges  of  the  Sun  on 
the  line  of  the  equator,  the  two  outer  from  the  west  and  the  cen- 
tral one  from  the  east  edge.  When  carefully  examined,  it  is 
seen  that  the  lines  are  slightly  displaced,  the  lines  of  the  central 
strip  are  to  the  left,  that  is,  to  the  violet,  of  those  in  the  outer 
strips.  From  a  microscropical  measurement  of  the  displace- 
ment AX  in  terms  of  the  wave-length  X  and  the  observed  veloc- 
ity V  of  light,  the  velocity  with  which  the  east  edge  of  the  Sun 
is  approaching  and  the  west  edge  receding  is  found  from  the 

formula  v  =  —^-  V  to  be  approximately  2  km.  per  second,  or 

nearly  4500  miles  per  hour.  It  follows  that  the  equatorial 
region  of  the  Sun  turns  on  its  axis  once  in  24.5  days.  The 
rotation  is  slower ,  for  higher  latitudes,  and  from  this  it  is 
evident  that  the  Sun  does  not  rotate  as  a  solid.  As  these 
differences  in  rotation  are  probably  vestiges  from  the  past,  a 
complete  knowledge  of  the  Sun's  rotation  is  important  in  the 
development  of  solar  theory. 

Some  of  the  lines  in  the  solar  spectrum  are  due  to  selective 
absorption  in  the  Earth's  atmosphere,  but  in  this  case  the  ab- 
sorbing matter  is  at  rest  relative  to  the  observer  and  the  lines 
of  terrestrial  origin  remain  undisplaced  in  spectra  from  the 
east  and  west  edges  of  the  Sun.  This  furnishes  a  means  of 
distinguishing  between  solar  and  terrestrial  lines.  Fig.  3, 
Plate  XXXVI,  reproduces  such  a  spectrum,  showing  the 
great  B  group  due  to  oxygen  in  the  Earth's  atmosphere,  the 
systematically  spaced  lines  occurring  in  the  deep  red.  These 
are  undisplaced  while  the  weaker  lines  of  solar  origin  are  all 
distinctly  shifted.  This  is  seen  especially  well  in  the  group  of 
four  faint  solar  lines  to  the  right  of  the  middle  of  the  portion 


(a)   Spots  and  granulations  of  the  photosphere.     Direct  photograph. 


-_ 

J*v^.  jfc 


(b)   Vortical  streaming  of  the  hydrogen  in  the  same  region. 
Spectroheliogram. 

PLATE  XXXVII. 
Solar  Observatory  Photographs. 


OUR  NEAREST  STAR,  THE  SUN  145 

of  the  spectrum  reproduced.  Here  also  the  central  strip  is 
from  the  east  edge  of  the  Sun's  disk  and  the  lines  of  solar  origin 
are  displaced  to  the  left,  that  is,  toward  the  violet. 

Another  application  of  the  Doppler  effect  is  the  study  of 
the  currents  in  the  solar  atmosphere  around  sun-spots.  It  is 
found  that  in  the  lower  levels  of  the  Sun's  atmosphere  the 
flow  from  spots  is  outward  along  the  Sun's  surface,  and 
inward  for  the  higher-lying  vapors  which  are  rushing  into 
spots  with  tremendous  cyclonic  whirls.  Since  the  Sun  is  a 
globe,  the  flow  outward  from  a  spot  near  the  Sun's  limb  is 
toward  the  observer  on  the  near,  and  from  the  observer  on 
the  far,  side  of  the  spot. 

In  Fig.  4,  Plate  XXXVI,  the  spectra  from  the  two  sides  of 
a  spot  are  shown  juxtaposed.  Along  the  line  of  juxtaposition 
the  lines  in  the  spectrum  from  the  near  side  are  displaced  to 
the  violet  and  those  from  the  far  side  are  displaced  to  the  red, 
indicating  in  both  cases  a  flow  outward.  The  displacements 
are  largest  for  the  faintest  or  low-level  lines.  They  become 
smaller  and  smaller  as  stronger  and  stronger  lines  are 
observed  until  for  the  strongest  lines  in  the  Sun's  spectrum, 
the  H  and  K  lines  of  calcium,  the  hydrogen  lines  and  the 
strongest  lines  of  sodium,  magnesium  and  iron,  the  displace- 
ments are  in  the  opposite  direction,  indicating  an  inflow  for 
the  high-level  vapors.  From  the  amount  of  displacement  it 
is  possible  to  determine  the  relative  distribution  of  the  con- 
stituents of  the  Sun's  atmosphere.  In  this  way  it  is  found 
that  the  vapors  of  the  heavy  and  rare  elements  occur  only  at 
the  lower  levels,  and  that  the  lighter  and  more  abundant 
substances  are  distributed  over  a  far  wider  range  of  altitude, 
some  of  them  forming  the  indefinite  boundary  of  the  Sun. 

The  surface  of  the  Sun,  the  photosphere,  ordinarily 
appears  to  the  unaided  eye  as  a  brilliant  disk  without  mark- 
ings of  any  kind,  but  when  photographed  or  observed  with  the 
telescope  the  surface  appears  distinctly  granular,  with  bright 
mottlings  upon  a  darker  background.  The  bright  patches, 
three  hundred  to  four  hundred  miles  in  diameter,  are  thought 
to  be  the  tops  of  rising  columns  of  hot  vapors.  Often  spots 
many  thousands  of  miles  across  are  seen,  each  with  a  dark 
center,  the  umbra,  surrounded  by  a  shaded  area,  the  penumbra. 


146  THE  ADOLFO  STAHL  LECTURES 

The  umbra  is  only  dark,  however,  in  comparison  with  the  bril- 
liant photosphere,  as  its  temperature,  though  lower  than  that  of 
its  surroundings,  is  comparable  with  the  highest  terrestrial 
temperatures.  These  features  may  be  recognized  in  the  repro- 
ductions of  Plate  XXXVII;  (a)  is  from  a  direct  photograph 
and  shows  granulations  and  spots ;  (b)  shows  the  same  region 
photographed  with  light  from  the  hydrogen  in  the  upper  solar 
atmosphere.  In  this  the  streaming,  whirling  movement  of  the 
hydrogen  gas  is  distinctly  seen  and  represents  a  cyclonic  storm 
of  vast  extent. 

The  umbra  of  a  sun-spot  is,  as  Hale  discovered,  a  power- 
ful magnetic  field.  This  is  shown  by  comparing  the  behavior 
of  the  spectrum  lines  in  spots  with  their  behavior  when  the 
radiating  vapor  is  in  a  strong  magnetic  field.  In  the  labora- 
tory, under  such  conditions,  many  lines  are  separated  into 
components  with  characteristic  properties,  the  Zeeman  effect. 
That  the  lines  behave  in  the  same  way  in  the  spectra  of  spots 
furnishes  positive  evidence  that  a  magnetic  field  exists  in  the 
umbra  of  a  sun-spot.  The  doubling  of  the  lines  when  light 
is  produced  in  a  magnetic  field  is  shown  in  (a)  Plate 
XXXVIII,  and  in  (b)  the  doubling  in  the  umbra  of  a  sun-spot. 

During  a  total  eclipse  of  the  Sun  great  red-colored 
prominences  are  often  seen  extending  many  thousands  of 
miles  beyond  the  limb.  These  are  mainly  clouds  of  hydrogen 
and  calcium  vapor  and  take  their  color  from  the  strong  red 
light  emitted  by  glowing  hydrogen.  These  are  now  recorded 
daily  by  covering  the  Sun's  image  with  a  circular  disk,  thus 
producing  an  artificial  eclipse,  and  photographing  them  by  the 
spectroheliograph,  an  instrument  by  which  the  surface  of  the 
Sun  and  its  surroundings  can  be  photographed  in  the  light  of 
a  selected  spectral  line.  In  (c)  and  (d),  Plate  XXXVIII,  a 
prominence  is  shown  photographed  in  this  way  by  Ellerman, 
with  a  long  exposure  to  get  the  detail  beyond  the  limb  and  with 
a  shorter  exposure  for  the  detail  of  the  portion  of  the 
prominence  projected  on  the  disk.  The  two  photographs  show 
that  certain  dark  markings  on  the  Sun's  disk  brought  out  only 
by  the  spectroheliograph  are  prominences  in  projection;  that 
is,  intervening  masses  of  cooler  vapor  high  above  the  visible 
surface  of  the  Sun.  These  absorb  from  the  transmitted  photo- 
spheric  light  more  light  of  their  own  rate  of  vibration  than 


OUR  NEAREST  STAR,  THE  SUN  147 

they  send  towards  the  Earth,  so  that  in  light  of  this  particular 
color  or  wave-length  they  appear  dark  against  the  hotter  and 
hence  brighter  background  of  the  disk.  It  is  the  characteristic 
property  of  the  spectroheliograph  to  "see"  the  Sun  photo- 
graphically in  the  light  of  any  selected  wave-length.  The 
illustrations  in  Plate  XXXVIII,  (c)  and  (d),  colored  the 
proper  shade  of  red,  that  of  the  red  light  of  hydrogen,  would 
represent  the  Sun  as  seen  by  an  eye  sensitive  to  this  particular 
color  and  blind  to  all  others. 

A  combination  of  two  photographs  obtained  by  the  spectro- 
heliograph is  reproduced  in  Plate  XXXIX.  One  shows  the 
Sun's  disk  taken  by  the  light  from  calcium  vapor  and  gives  the 
distribution  of  this  particular  substance  in  the  solar  atmos- 
phere at  that  time,  the  other  records  the  accompanying 
prominences  then  projecting  beyond  the  Sun's  visible  edge. 
These  are  the  bright  red  protuberances  which  form  the  most 
striking  feature  when  the  Sun  is  covered  by  the  Moon  during 
a  total  eclipse.  On  the  disk,  bands  of  bright  flocculi  are  shown 
in  the  two  sun-spot  belts,  one  on  each  side  of  the  Sun's 
equator.  These  areas  change  in  number,  size,  and  configura- 
tion, following  variations  in  solar  activity,  and  are  always 
conspicuous  in  the  neighborhood  of  sun-spots.  The  Sun  is 
by  no  means  in  a  quiescent  state.  Variation  in  its  activity  is 
indicated  not  only  by  the  increase  or  decrease  in  the  size  and 
number  of  spots,  faculae,  prominences,  and  flocculi,  the 
recording  of  which  is  now  a  matter  of  daily  routine  at  a  solar 
observatory,  but  also  by  the  movements,  sometimes  on  a 
tremendous  scale,  in  its  enveloping  atmosphere.  The  ordinary 
speed  of  outflow  of  low-lying  vapors  from  spot  centers  along 
the  solar  surface  is  fifty  to  a  hundred  miles  per  second, 
velocities  of  an  order  of  magnitude  not  approached  in  the 
Earth's  atmosphere. 

A  striking  illustration  of  the  rapidity  of  movement  in  the 
upper  regions  of  the  solar  atmosphere  is  shown  in  Plate  XL. 
A  dark,  that  is,  a  relatively  cool,  cloud  of  hydrogen  had  been 
observed  for  some  days  projected  against  the  glowing  photo- 
sphere. It  was  apparently  motionless,  but  one  day  a  series 
of  nine  hydrogen  spectroheliograms  was  made  in  quick  suc- 
cession upon  a  single  photographic  plate.  It  happened  that 
just  then  the  cloud  was  caught  in  the  current  and  was  rapidly 


148  THE  ADOLFO  STAHL  LECTURES 

drawn  into  a  neighboring  spot  or  pair  of  spots.  It  attained  a 
velocity  of  nearly  a  hundred  miles  a  second  and  when,  after 
development  of  the  plate,  another  trial  was  made  all  trace  of 
it  had  disappeared.  The  real  importance  of  the  observation 
is  that  it  showed  the  direction  of  movement  along  the  arms  of 
the  spiral  structure  that  occurs  around  sun-spots  upon  hydro- 
gen spectroheliograms  (Plate  XXXVII),  a  question  upon  which 
evidence  at  that  time  was  not  conclusive.  Later  developments 
in  methods  of  observation,  applying  the  Doppler  principle, 
though  less  spectacular,  enable  one  to  make  the  observation  for 
any  spot  when  it  is  near  the  Sun's  visible  edge. 

As  it  is  possible  by  working  with  the  slit  of  the  spectro- 
graph  close  to  the  edge  of  a  large  image  of  the  Sun  to  see 
and  to  photograph  in  full  sunlight  the  "flash"  spectrum,  the 
bright  lines  given  by  the  Sun*s  gaseous  atmosphere  when  the 
white  disk  is  just  covered  by  the  Moon  at  a  total  eclipse,  there 
remains  but  one  of  the  recognized  solar  phenomena  that 
requires  for  its  observation  the  conditions  obtaining  only  at  a 
total  solar  eclipse,  namely,  the  corona,  an  extensive  halo  of 
greenish  pearly  light  so  faintly  luminous  that  the  sunlight 
diffused  in  the  Earth's  atmosphere  renders  it  invisible  except 
when  that  light  is  cut  off  by  the  Moon  at  a  total  eclipse.  The 
coronal  light  is  thought  to  arise  partly  from  sunlight  by  a 
kind  of  dust-fog  around  the  Sun,  and  partly  from  a  hypo- 
thetical element,  coronium,  giving  the  characteristic  green  ray 
that  corresponds  to  nothing  known  in  the  Sun  or  upon  the 
Earth.  This  lends  the  corona  a  peculiar  interest  and  together 
with  the  uncertainties  concerning  its  nature  and  relationship 
to  the  Sun  must  for  a  long  time  give  it  prominence  in  the 
program  of  eclipse  observations. 

Numerous  efforts  have  been  made  to  discover  connections 
between  changes  in  the  Sun  and  terrestrial  phenomena.  Sun- 
spots,  faculae,  and  prominences  increase  together  to  a  maxi- 
mum number,  decrease  to  a  minimum,  then  rise  again  to  a 
maximum  in  regular  sequence,  that  is,  they  show  a  definite 
periodicity.  The  question  may  be  raised,  Are  there  phenomena 
on  the  Earth  that  run  the  same  periodic  courses?  If  ter- 
restrial changes  manifest  the  same  orderly  sequence  over  a 
long  period  of  years  we  would  be  justified  in  assuming  a 
connection  between  the  solar  and  such  terrestrial  phenomena. 


a  b 

(a)  Doubling  of  lines  in  the  magnetic  field. 

(b)  Doubling  of  line  in  the  umbra  of  a  sun-spot. 


(c)  Detail  of  prominence  beyond  the  limb. 

(d)  Detail  of  its  projection  in  the  disk. 

Photographs  by  F.  Ellerman. 


PLATE  XXXVIII. 


OUR  NEAREST  STAR,  THE  SUN 


149 


Sun-spots  have  been  observed  for  a  hundred  and  fifty  years. 
When  the  spot  numbers  are  plotted  for  the  different  years  the 
resulting  curve  shows  that  they  occur  in  cycles  and  that  the 
average  period  of  the  cycle  from  maximum  to  maximum  is 
11.1  years.  The  magnetic  elements  of  the  Earth  show,  aside 
from  the  secular  and  regular  daily  variations,  irregular 
fluctuations  in  intensity;  the  so-called  magnetic  storms  are 
examples  of  extremely  vigorous  disturbances  of  this 
character.  When  the  sun-spot  and  magnetic-variation  curves 
are  compared,  they  are  found  to  be  identical  in  period  and  the 
peculiarities  in  one  are  matched  by  similar  peculiarities  in  the 
other.  No  one  questions  their  intimate  connection,  but  when 


FIG.  10.     COMPARISON  OF  RAINFALL  WITH  SUN-SPOTS. 

<• 

an  effort  is  made  to  correlate  the  weather,  the  rainfall  for 
example,  with  sun-spots  it  has  not  as  yet  been  possible  to 
establish  any  well  defined  relation.  It  may  be  interesting  to 
compare  the  rainfall  in  California  with  the  sun-spot  curve. 
Records  at  San  Francisco,  Stockton,  and  Sacramento  are 
available  for  nearly  seventy  years.  Such  a  comparison  is 
shown  in  the  curves  in  Fig.  10.  At  once  it  is  seen  that  the 
years  of  maximum  rainfall  coincide  with  neither  the  maximum 
nor  the  minimum  of  the  sun-spot  curve.  The  danger  of 
basing  a  conclusion  upon  too  limited  data  is  illustrated  in  the 
two  short  curves  in  Fig.  10.  Curve  (a),  a  composite  for  the 
last  35  years,  shows  an  approximation  to  similarity  with  the 
spot  curve,  while  curve  (b),  for  the  first  35  years,  shows  com- 


150  THE  ADOLFO  STAHL  LECTURES 

plete  dissimilarity.  No  one  has  been  able  to  suggest  any  valid 
ground  for  expecting  a  direct  connection  between  sun-spots  and 
local  rainfall;  but  when  it  is  found  by  observations  that  there 
are  changes  in  the  amount  of  <[iea)  sent  to  us  from  the  Sun's 
abounding  store,  we  would  seem  to  be  justified  in  expecting  to 
find  a  direct  relation  between  terrestrial  temperatures  and 
variations  in  solar  radiation,  as  the  Earth's  temperature  is  a 
function  of  the  Sun's  heat  emission.  We  would  expect  to 
find  a  general  rise  in  temperature  with  increased  solar  radia- 
tion, but  even  here  the  matter  is  not  so  simple.  During  a 
sun-spot  maximum  the  Sun  sends  us  three  or  four  per  cent  more 
fieapthan  during  the  minimum.  The  spots  are  not  directly  con- 
cerned in  this  increased  radiation,  they  are  only  symptoms  of 
the  greater  activity  of  the  Sun.  The  solar  gases  are  in  a  more 
turbulent  state  and  bring  more  (JKgt)  from  the  hot  interior  to 
the  surface  during  the  periods  of  increased  activity.  This 
change  of  three  or  four  per  cent  is  distributed  over  a  space  of 
five  or  six  years  and  hence  is  slow  in  producing  its  effect,  but 
fluctuations  of  five  or  six  per  cent  that  run  their  course  in  a 
week  or  ten  days  are  shown  by  the  Smithsonian  observations. 
The  temperatures  at  fifty  stations  well  distributed  over  the 
Earth  have  been  correlated  by  Dr.  Clayton  with  the  indicated 
short-period  fluctuations  in  the  solar  radiation.  The  results  are 
surprising.  In  the  equatorial  regions  the  temperatures  rise 
with  increased  solar  radiation,  but  in  the  Earth's  temperate 
zones  the  temperatures  fall.  At  Pilar,  Argentina,  increase  of 
temperature  followed  increase  of  solar  radiation  and  reached 
the  maximum  effect  in  one  or  two  days,  while  at  San  Diego, 
California,  decrease  of  temperature  followed  increase  of  solar 
radiation,  and  the  maximum  effect  occurred  after  three  or  four 
days.  Manifestly  secondary  causes  are  set  in  motion  which 
in  part  mask  the  direct  solar  action  in  the  temperate  zones. 
The  Sun  being  more  nearly  overhead  in  the  equatorial  regions, 
the  influence  of  increased  radiation  is  there  more  quickly  felt, 
the  temperature  of  the  atmosphere  is  increased  and  the 
abnormally  heated  air  rises  and  overflows  the  temperate  zones, 
producing  conditions  that  disturb,  in  a  way  unknown  as  yet, 
the  blanketing  effect  of  the  atmosphere.  A  similar  paradoxical 
result  appears  in  the  lower  temperatures  of  the  world  in 


PLATE  XXXIX. 
COMBINED  PHOTOGRAPHS  OF  PROMINENCES  AND  FLOCCULI. 

Solar  Observatory  Photographs. 


OUR  NEAREST  STAR,  THE  SUN  151 

general  at  sun-spot  maximum  than  at  minimum,  though  the 
solar  radiation  is  greater  at  sun-spot  maximum.  The  observed 
lowering  in,  temperature  is  about  one  degree  Fahrenheit  while 
the  increased  radiation  of  the  Sun  indicates,  according  to 
Abbot,  a  rise  of  some  three  or  four  degrees.  The  variations  in 
the  amount  of  heat  given  out  by  the  Sun  that  run  their  course 
in  a  few  days  and  are  followed  by  observable  changes  in 
temperature  over  definite  regions  of  the  Earth  suggest  the 
possibility  of  being  able  in  the  near  future  to  forecast  related 
terrestrial  conditions  over  extended  regions  days  in  advance 
of  their  occurrence. 

The  measurement  of  the  solar  constant,  the  total  intensity 
of  solar  radiation  outside  the  Earth's  atmosphere  at  the 
Earth's  mean  distance  from  the  Sun,  as  made  by  the  observers 
of  the  Smithsonian  Institution  at  the  Washington,  Mount  Wil- 
son and  Mount  Whitney  stations,  is  1.95  calories  per  square 
centimeter  per  minute  and  it  is  thought  that  future  investiga- 
tion will  make  no  considerable  change  in  this  value.  The 
amount  of  energy  represented  by  this  radiation  is  difficult  of 
conception.  Assuming,  as  we  have  reason  to  do,  that  the  Sun 
radiates  equally  in  all  directions,  we  can  easily  calculate  the 
total  emission,  as  it  is  1.95  calories  per  minute  on  each  square 
centimeter  of  a  sphere  whose  radius  is  the  mean  distance  of 
the  Earth  from  the  Sun,  that  is,  93,000,000  miles,  or  15  X  1012 
centimeters. 

Total  emission  =  1.95  X  4  (15  X  1021)2  calories  per  min- 
ute. This  is  sufficient,  as  Abbot  calculates,  to  melt  a  layer  of 
ice  426  feet  thick  in  a  year.  A  layer  426  feet  thick  over  the 
cross-section  of  the  Earth  is  equivalent  to  a  layer  106.5  feet 
over  its  surface,  so  that  we  can  say  that  the  heat  received  by 
the  Earth  in  a  year  is  sufficient  to  melt  a  surrounding  shell  of 
ice  106.5  feet  thick.  Abbot  further  calculates  that  the  melting 
in  a  year  of  a  shell  of  ice  426  feet  thick  surrounding  the  Sun 
at  the  Earth's  mean  distance  would  represent  as  many  heat 
units  as  the  burning  of  4  X  1023  tons  of  anthracite  coal,  or  a 
mass  of  coal  60  times  the  mass  of  the  Earth. 

The  great  terrestrial  sources  of  heat  are  combustion,  the 
transformation  into  heat  of  electrical  energy  obtained  from 
water  power,  the  disintegration  of  radio-active  elements,  and 


152  THE  ADOLFO  STAHL  LECTURES 

the  Earth's  internal  store.  If  we  try  to  account  for  the  Sun's 
heat  by  combustion  we  reach  an  absurdly  small  result  for  the 
life  of  the  sun.  We  have  just  seen  that  the  yearly  output  of 
heat  is  equivalent  to  that  from  the  burning  of  4  X  1023  tons  of 
coal.  If  the  Sun  were  composed  of  pure  coal  its  combustion 
would  supply  the  heat  loss  only  for 
2.19  X  1027 


4  X  10" 
a  moment  only  in  the  life  history  of  the  Sun-Earth  system. 

It  has  been  suggested  that  the  maintenance  of  the  solar 
radiation  is  clue  to  the  continued  fall  of  meteoric  matter  into 
the  Sun.  A  mass  coming  from  an  infinite  distance  would 
acquire  a  velocity  of  610  kilometers  or  385  miles  per  second  at 
the  surface  of  the  Sun  and  when  brought  to  rest  would  dis- 
engage 6,000  times  as  much  heat  as  would  be  produced  if  it 
were  coal  burning  in  oxygen.  To  compensate  for  the  loss  of 
radiation  would  require  that  22  pounds  of  matter  fall  upon 
each  square  yard  of  the  Sun's  surface  per  hour.  This  would 
increase  the  diameter  of  the  Sun  so  slowly  that  35,000,000 
years  must  elapse  before  the  increase  would  attain  one  second 
of  arc.  It  would,  however,  increase  the  mass  of  the  Sun  to 
such  an  extent  that  the  effect  could  not  escape  detection. 
Rosier  calculates  that  in  the  last  2,000  years  the  accumulation 
would  have  been  sufficient  to  change  the  orbital  motion  of  the 
Earth  by  one-eighth  of  a  year,  a  change,  needless  to  say,  that 
has  not  occurred.  Moreover,  few  meteors  coming  from  inter- 
stellar space  would  fall  into  the  Sun,  as  most  of  them  would 
circulate  around  it  as  comets  do. 

A  source  of  heat  that  has  been  very  generally  admitted 
since  its  suggestion  by  Helmholtz,  is  the  gravitational  attrac- 
tion of  the  Sun  upon  its  own  material,  as  a  gradual  falling  of 
the  Sun's  substance  toward  the  center  would  transform  the 
potential  energy  of  gravitation  into  heat.  The  estimates  of 
the  energy  available  in  the  past  from  this  source  are  based 
upon  the  contraction  of  the  Sun  to  its  present  size  from  a 
diameter  exceeding  that  of  the  orbit  of  Neptune,  the  outer- 
most known  member  of  the  solar  system.  The  energy 
supplied  by  this  contraction  would  have  sustained  the  present 
rate  of  radiation  for  approximately  25,000,000  years.  Accord- 


OUR  NEAREST  STAR,  THE  SUN  153 

ing  to  Newcomb  the  Sun  will  have  shrunk  to  half  its  present 
diameter  in  7,000,000  years  and  will  be  unable  to  furnish  heat 
sufficient  to  support  life  as  we  know  it  for  more  than 
15,000,000  years. 

Though  the  gravitational  contraction  of  the  Sun  is 
regarded  as  a  real  source  of  energy,  it  is  generally  admitted 
that  it  alone  is  not  sufficient  to  account  for  radiation  through 
the  enormous  periods  of  time  required  for  the  geological 
transformation  of  the  Earth.  In  the  effort  to  meet  this 
recognized  difficulty  the  suggestion  has  been  made  that  the 
solar  radiation  was  less  intense  during  past  ages  than  at 
present,  the  deficit  being  supplied  by  the  inherent  heat  of  the 
Earth  or  by  receiving  heat  from  a  large  solid  angle  subtended 
by  a  greatly  extended  nebular  Sun.  And  since  the  discovery 
of  the  liberation  of  energy  by  the  breaking  up  of  radioactive 
substances,  much  attention  has  been  given  to  the  suggestion 
that  the  presence  of  such  substances  in  the  Sun  would  assist 
in  maintaining  the  solar  radiation  and  give  it  sufficient 
duration  to  meet  the  requirements  of  geological  transforma- 
tion. Direct  proof  of  their  presence  in  the  Sun  is  lacking, 
though  the  occurrence  of  helium  and  lead  in  the  Sun,  products 
of  the  disintegration  of  radium,  may  be  taken  as  indicative  of 
their  possible  presence.  That  the  lines  of  radioactive  elements 
do  not  occur  in  the  solar  spectrum  is  not  surprising  in  view  of 
their  high  atomic  weights.  If  radium  and  its  parent  element, 
uranium,  do  exist  in  the  Sun,  they  are  probably  at  a  very  low 
level  in  the  solar  atmosphere  and  their  lines  would  conse- 
quently be  extremely  faint  or  absent.  The  whole  question  is 
one  of  extreme  difficulty  and  has  not  as  yet  received  a 
satisfactory  solution. 

The  outer  portions  of  the  Sun  are  certainly  gaseous.  This 
is  shown  by  the  presence  of  lines  in  its  spectrum,  since  gases 
only  can  give  a  line  spectrum.  The  photosphere  forms  the 
visible  disk  of  the  Sun  and  is  the  source  of  the  continuous 
spectrum.  Upon  the  constitution  of  the  photosphere  astrono- 
mers are  not  in  agreement.  Some  consider  it  a  cloudy  layer 
similar  to  clouds  in  our  own  atmosphere,  but  while  the  ter- 
restrial clouds  consist  of  minute  water  droplets  suspended  in 
the  air,  the  solar  clouds  are  supposed  to  be  the  condensed  vapors 


154  THE  ADOLFO  STAHL  LECTURES 

of  unknown  substances  floating  in  the  atmosphere  of  incon- 
densable vapors.  According  to  the  investigation  of  Abbot  the 
temperature  of  the  photosphere  can  not  be  lower  than  10,500° 
F.,  and  probably  not  less  than  11,500°  F.  Moissan  found  that 
all  known  elements  volatilize  at  a  temperature  of  3,500°  C.  or 
6,300°  F.  In  view  of  these  observational  results  it  is  thought 
by  other  solar  physicists  that  clouds  can  not  exist  in  the  Sun's 
atmosphere  and  that  the  continuous  spectrum  originates  in  the 
lower  and  denser  layers  under  conditions  in  which  gases  would 
give  a  continuous  spectrum. 

As  to  the  state  of  matter  in  the  interior  of  the  Sun  we 
know  nothing  by  observation,  and  here  again  the  astronomers 
have  different  opinions.  All  agree  that  the  temperatures  in 
the  Sun's  interior  are  vastly  higher  than  the  surface  tempera- 
ture, reaching  many  millions  of  degrees,  and  that  the  pressures 
due  to  the  Sun's  gravitation  are  also  tremendous  near  the  core. 
As  we  know  nothing  experimentally  of  the  behavior  of  matter 
under  such  extremes  of  temperature  and  pressure,  the  field  is 
open  for  individual  opinion.  In  view  of  the  low  average 
density  of  the  Sun,  one-fourth  that  of  the  Earth  or  1.4  times 
that  of  water,  it  is  clear  that  very  far  down  below  the  surface 
the  Sun  must  still  be  gaseous.  Those  who  consider  that  the 
Sun  may  have  a  solid  or  liquid  core  deduce  their  conclusions 
from  the  enormous  pressure  existing  there.  Those  who 
believe  that  the  whole  interior  is  gaseous  look  at  the  question 
more  from  the  point  of  view  of  temperature.  Though  air, 
hydrogen,  and  helium,  the  most  refractory  of  the  elements, 
can  be  liquefied  under  pressures  available  in  the  laboratory, 
they  must  at  the  same  time  be  below  certain  critical  tempera- 
tures before  any  pressure,  however  great,  can  liquefy  them. 
As  the  temperature  in  every  part  of  the  Sun  is  above  the 
critical  temperature  of  every  known  substance,  the  prevalent 
opinion  is  that  the  whole  interior  of  the  Sun  is  gaseous. 

When  the  Sun  is  considered  among  a  universe  of  stars,  it 
is  only  one  among  hundreds  of  millions.  The  distance  from 
its  nearest  known  neighbor  is  so  great  that  it  transcends  the 
imagination.  In  terms  of  the  velocity  of  light  its  distance  is 
about  4.4  light-years,  that  is,  the  distance  traversed  by  light  in 
4.4  years  with  a  velocity  of  186,000  miles  a  second.  There  are 


PLATE  XL. 
HYDROGEN  FLOCCULUS  DRAWN  INTO  SUN-SPOT. 

Solar  Observatory  Photographs. 


OUR  NEAREST  STAR,  THE  SUN  155 

perhaps  thirty  or  forty  stars  within  a  radius  of  four  times  this 
distance.  It  is  evident  that  in  a  sense  we  are  quite  alone  in 
space  even  with  a  hundred  million  other  Suns.  We  speak  of 
the  fixed  stars,  but  this  is  a  misnomer,  as  they  are  all  in  rapid 
motion,  but,  because  of  their  great  distance,  movement  can  only 
be  detected  by  measurements  of  the  highest  precision.  Our  Sun 
is  no  exception,  as  it  is  sweeping  through  space  with  a  velocity 
of  twelve  and  a  half  miles  per  second,  a  speed  that  carries 
the  Sun  and  its  attendant  train  of  planets  over  a  million  miles 
a  day,  so  that  when  the  Earth  has  made  a  complete  revolution 
around  the  Sun,  it  is  still  385,000,000  miles  from  where  it  was 
the  year  before.  With  all  this  speed  it  would  require  70,000 
years  to  reach  the  nearest  star,  even  if  we  were  traveling  in 
that  direction. 

The  question  of  very  great  interest  is,  How  did  our  solar 
system  come  into  existence  and  what  will  be  its  future?  The 
evolution  of  the  Sun  takes  place  so  slowly  that  no  change  has 
been  noted  in  historical  times.  We  cannot  hope  to  solve  its 
past  nor  to  foretell  its  future  evolution  from  observations  on 
the  Sun  alone;  but  the  Sun  is  one  among  the  other  stars  and 
these  apparently  represent  a  series  of  types  in  a  progression 
from  a  nebular  stage  to  a  dead  or  dying  Sun.  When  from  a 
knowledge  gained  from  the  study  of  their  spectra  and  other 
characteristics  the  various  stages  in  stellar  evolution  are 
found,  it 'will  be  possible  from  its  spectrum  to  locate  the  Sun 
in  the  series  of  evolving  stars,  and  both  its  future  and  its  past 
may  be  determined.  The  story  is  written  in  the  ether  of  space 
and  must  be  learned  from  the  interpretation  of  the  records 
made  by  the  spectroscope.  This  is  why  the  modern  astrono- 
mer speaks  and  writes  so  continually  of  the  spectrum  and  its 
teachings,  and  the  layman  who  wishes  to  know  the  basis  and 
not  merely  the  results  of  the  astronomer's  conclusions  will 
find  it  of  great  assistance  to  familiarize  himself  with  the 
principles  of  spectrum  analysis. 

The  immediate  province  of  a  solar  observatory  is  to  solve 
as  far  as  possible  the  problems  relating  to  our  Sun.  To  take 
this  citadel  of  the  sky  three  lines  of  attack  are  open  to  us,  all 
converging  upon  the  central  objective.  First,  the  direct 
attack  upon  the  Sun  itself.  This  offers  great  opportunities 


156  THE  ADOLFO  STAHL  LECTURES 

and  the  hope  of  immediate  gains ;  moreover,  as  it  is  our  near- 
est star,  the  minuter  details  of  stellar  character  can  be  studied 
with  great  advantage  through  the  Sun.  Second,  the  Sun  is 
only  one  among  a  universe  of  similar  suns,  so  that  the  broader 
question  of  the  relation  to  the  sidereal  system  and  the 
evolutionary  history  of  the  Sun  are  most  hopefully  approached 
through  a  study  of  the  distant  stars.  Third,  the  student  of 
the  Sun  must  ever  have  his  feet  upon  the  solid  Earth,  his 
observations  must  be  checked  and  interpreted  and  often 
directed  by  investigations  in  the  physical  laboratory,  which 
therefore  forms  an  essential  adjunct  to  the  observatory. 
Through  the  coordination  of  these  three  modes  of  approach 
and  the  harmonious  interaction  between  them,  the  great 
advances  of  the  immediate  past  have  been  made  and  far 
greater  gains  of  the  future  may  be  confidently  hoped  for,  and 
the  dream  of  the  savage  and  the  civilized  man  as  pictured  by 
Wells  in  The  World  Set  Free  may  yet  be  fulfilled.  He  says 
of  the  savage: 

Man  began  to  think.  There  were  times  when  he  was  full,  when 
his  lusts  and  his  fears  were  all  appeased.  He  watched  the  streaming 
river  and  wondered  from  what  bountiful  breast  this  incessant  water 
came;  he  blinked  at  the  Sun  and  dreamt  that  perhaps  he  might  snare 
it  and  spear  it  as  it  went  down  to  its  resting  place  amidst  the  distant 
hills. 

Of  the  twentieth-century  boy  who  has  just  had  his 
imagination  fired  by  a  lecture  on  the  wonders  of  radium,  he 
writes : 

He  made  his  way  to  the  top  of  Arthur's  Seat  and  there  he  sat  for 
a  long  time  in  the  golden  evening  sunshine,  still,  except  that  ever  and 
again  he  whispered  to  himself  some  precious  phrase  that  stuck  in  his 
mind.  "If,"  he  whispered,  "if  only  we  could  pick  that  lock."  He 
seemed  to  wake  up  at  last  out  of  his  entrancement  and  the  red  Sun 
was  before  his  eyes.  Into  his  mind  came  a  strange  echo  of  that  ances- 
tral fancy,  that  fancy  of  a  Stone  Age,  dead  and  scattered  bones  among 
the  drift  two  thousand  years  ago.  "Ye  auld  thing,"  he  said,  and  his 
eyes  v/ere  glistening  and  he  made  a  kind  of  grabbing  gesture  with  his 
hand,  "Ye  auld  red  thing.  .  .  .  We'll  have  ye  yet." 


PLATE  XLI.     THE  GREAT  NEBULA  IN  ORION. 
Photographed  by  J.  E.  Keeler,  Crossley  Reflector,  Nov.  16,  1898. 


NEWS  FROM  THE  STARS1 

By  ROBERT  G.  AITKEN 

Like  the  Athenians  in  the  days  of  St.  Paul,  we  all  delight  to 
tell  or  hear  of  some  new  thing.  "What's  the  news?"  is  a 
standard  form  of  greeting  and  few  of  us  can  pass  a  bulletin 
board  or  a  newsboy  shouting  "extra"  without  stopping  to  get 
the  news.  And  marvelous  indeed  is  the  organization  that 
makes  it  possible  for  us  to  learn  each  day  the  more  important 
items  of  news  from  every  part  of  the  civilized  world.  Whether 
it  is  that  Steffanson  has  reached  Fort  Yukon  after  his  long 
stay  in  the  Arctic  regions,  that  Guatemala  has  been  visited  by 
a  disastrous  earthquake,  or  that  General  Allenby  has  entered 
Jerusalem  on  foot,  the  agents  of  the  Associated  Press  have 
noted  the  fact  almost  before  the  event  and  we  read  of  it  next 
morning  in  our  daily  paper. 

At  the  present  time,  of  course,  the  news  we  are  all  most 
eager  to  hear  is  the  news  from  "over  there,"  and  in  this  the 
astronomer  is  as  keen  as  the  most  "practical"  man  of  business. 
I  am  well  aware  that  the  latter  sometimes  regards  the  astrono- 
mer with  a  certain  air  of  good-humored  tolerance,  as  a  man 
who  walks  with  his  head  in  the  clouds  and  his  eyes  fixed  upon 
the  stars,  oblivious  of  the  ordinary,  or  even  the  extraordinary, 
affairs  of  our  common  daily  lives.  And  it  would  indeed  seem 
that  if  any  were  to  be  unaffected  by  the  present  war  it  might 
well  be  a  little  company  of  men  dwelling  upon  a  more  or  less 
isolated  mountain  top,  engaged  in  the  purely  scientific  study  of 
the  stars. 

But  let  me  bear  witness  that  we  are  united  with  you  in  one 
brotherhood  in  our  love  of  country  and  of  righteousness,  and 
that  we  are  striving  even  as  you  to  do  our  part  toward  making 
justice  and  right  prevail  upon  the  Earth.  Every  man,  woman, 
and  child,  even  to  the  month-old  baby,  in  our  little  community 

1  Delivered  January   11,   1918. 


158  THE  ADOLFO  STAHL  LECTURES 

on  Mount  Hamilton  is  a  member  of  the  American  Red  Cross ; 
every  girl  and  woman  is  giving  every  possible  minute  to  knit- 
ting and  sewing  for  the  Red  Cross ;  every  employee  of  the 
Lick  Observatory  holds  at  least  one  Liberty  Bond,  every 
household  is  intelligently  and  conscientiously  conserving  food 
and  fuel ;  our  little  community  has  "gone  over  the  top"  in  every 
"drive''  for  funds,  beginning  with  the  appeals  for  relief  long 
before  the  first  Red  Cross  drive  last  spring.  And  that  is  the 
least  of  it.  Practically  every  family  has  near  relatives  at  the 
front,  and  four  of  our  boys,  sons  of  the  three  astronomers 
who  have  boys  old  enough  to  serve,  are  volunteers  in  the  active 
military  service  of  their  country.  Two  are  in  France  at  this 
moment,  Lieutenants  in  the  Engineer  Corps  and  in  the  Avia- 
tion Service ;  one  is  on  board  a  man-of-war,  and  the  fourth  is 
in  the  Marine  Corps.  Yes,  I  think  I  may  say  that  the  astrono- 
mers on  Mount  Hamilton  are  interested  in  the  news — all  the 
news — bearing  in  any  way  upon  the  war.2 

It  is  our  personal  duty,  meanwhile,  quietly  to  continue  get- 
ting the  news  from  the  stars  and  making  it  known  to  those 
who  may  be  interested.  In  this  work  we  cannot  rival  our 
friends  of  the  Associated  Press  in  promptness.  However  alert 
we  may  be,  however  quick  to  seize  and  decipher  the  messages 
flashed  to  us  with  the  speed  of  light  from  "the  marches  and 
strongholds  of  space,"  our  news  Jags  far  behind  the  event. 
You  were  doubtless  reminded  of  that  fact  if  you  read  an  article 
that  appeared  in  one  of  the  San  Francisco  papers  one  morning 
in  December.  The  headlines  ran  : 

2  Although  this  paragraph  has  no  definite  relation  to  the  subject  of  "News 
from  the  Stars,"  it  is  allowed  to  stand  as  an  expression  of  the  attitude  of  the 
Lick  Observatory  community  to  the  war.  In  reading  it,  the  date  (January  11, 
1918)  must  be  held  in  mind.  Later  on  the  Marine  mentioned  crossed  to  France 
and  saw  hard  service  in  the  front  line;  and  two  other  boys,  sons  of  astronomers 
in  the  Observatory,  entered  upon  active  military  service  as  volunteers.  One  of 
the  two  went  to  Italy  and  rendered  valiant  service  in  the  ambulance  corps;  the 
other  was  commissioned  Second  Lieutenant  of  Infantry  and  detailed  as  instructor 
to  a  Trairiing  Camp  in  this  country.  Indeed  every  male  graduate  of  the  little 
grammar  school  on  Mount  Hamilton  volunteered  for  war  work. 

The  responses  of  the  community  to  the  later  Liberty  Bond,  Red  Cross  and 
other  "drives"  were  as  prompt  and  generous  as  those  to  the  earlier  ones.  In  every 
instance  the  amount  asked  for  was  oversubscribed — sometimes  four  to  eight-fold — 
on  the  opening  day  of  the  drive.  For  a  more  complete  statement  see  a  note  by 
Dr.  W.  W.  Campbell  in  the  Publ.  Astron.  Soc.  Pac.,  30,  353,  1918. 


NEWS  FROM  THE  STARS  159 

THIS  NEWS  IS  LATE,  BUT  HERE  IT  IS  AT  LAST 

EXTRA  !   EXTRA  !   ALL  ABOUT   BIG  DISASTERS   OF  20,000,000 

YEARS  AGO 


THREE  SUNS  BLOWN  UP 


Information  Reaches  Earth  Finally  as  Tiny  Specks  on  Photo- 
graphic Plate. 


The  article  was  based  upon  a  Lick  Observatory  Bulletin 
announcing  the  discovery  by  Dr.  Curtis  of  three  novae,  new 
stars,  in  spiral  nebulae,  and  barring  the  statement  of  the  "blow- 
ing up"  of  three  suns  and  of  a  few  other  details  was  accurate 
enough  and  certainly  very  interesting  reading.  A  cipher  or 
two  might  perhaps  be  dropped  from  the  number  of  years  given 
in  the  headlines  quoted,  but,  at  best,  the  news  was  a  very  long 
time  indeed  in  reaching  us,  measured  by  the  standards  of  our 
human  experience.  I  shall  tell  you  more  about  this  item  of 
news  a  little  later  on,  but  just  now  I  want  to  ask  you  to  fix  your 
attention  on  the  stars  which  shine  upon  us  in  the  early  hours 
of  these  winter  evenings  when  we  face  the  south  and  look  up 
into  the  sky. 

There  are  few  regions  of  the  starry  heavens  more  attractive 
to  the  unaided  eye  than  the  one  now  spread  before  you.  High 
in  the  sky,  near  the  zenith,  is  the  little  group  of  the  Pleiades ; 
south  and  to  the  east  from  them  stand  the  Hyades,  with  ruddy 
Aldebaran  for  their  leader ;  still  farther  southeast  is  Orion ;  and 
towards  the  southeastern  horizon,  Sirius,  the  brightest  star  in 
the  sky.  East  and  a  little  north  from  the  red  star  Betelgeux, 
Alpha  Orionis,  shines  Procyon,  and  north  and  slightly  east 
of  Procyon  the  twin  stars,  Castor  and  Pollux.  The  great 
planet  Jupiter,  between  the  Pleiades  and  Aldebaran,  and 
Saturn,  low  in  the  eastern  sky,  are  added  attractions  during  the 
present  winter. 

Beautiful  as  it  is  to  the  unaided  eye,  every  increase  in 
optical  power  as  we  apply  the  telescope  to  the  various  parts  of 
this  section  of  the  sky  brings  out  new  wonders.  Not  only  is 
the  apparent  number  of  stars  increased  beyond  our  power  to 


160  THE  ADOLFO  STAHL  LECTURES 

count,  but  many  of  them  are  found  to  be  double  or  multiple ; 
others,  to  be  surrounded  by  those  cloud-like  masses  of  light 
which  we  call  nebulae.  Theta  Orionis,  the  middle  star  in  the 
sword  of  Orion,  for  example,  which,  indeed,  is  hazy  to  the  eye 
alone,  is  now  seen  to  be  a  nebulous  mass  entwined  about  a  little 
group  of  stars.  This  object,  commonly  known  as  the  Great 
Nebula  in  Orion,  is  in  fact  one  of  the  most  remarkable  in  the 
whole  heavens  and  it  is  one  about  which  we  have  recently  been 
finding  out  some  new  facts  which  I  am  sure  will  be  of  interest 
to  you. 

To  realize  their  significance  it  will  be  necessary  to  glance 
briefly  at  the  history  of  this  nebula  as  revealed  by  the  telescope. 
As  long  ago  as  1656  Huyghens,  the  great  Dutch  astronomer 
who  first  solved  the  problem  of  Saturn's  puzzling  aspect 
as  viewed  through  early  telescopes,  saw  three  of  the  stars  in 
the  little  group  of  the  now  familiar  Trapezium ;  the  fourth  was 
certainly  known  in  HerscheFs  time,  and,  later,  fainter  com- 
panion stars  were  added  to  two  of  the  four.  One  or  two 
excessively  faint  stars  within  the  Trapezium  were  discovered 
by  Alvan  Clark  and  by  Barnard  with  our  36-inch  refractor : 
and  Frost  and  Adams,  at  the  Yerkes  Observatory,  found  the 
brightest  star  of  the  Trapezium  to  be  a  spectroscopic  binary 
system.  Merely  as  a  star  group,  then,  Theta  Orionis  is  a 
wonderful  object;  a  group  of  suns  forming  a  single  physical 
system  of  a  size  so  vast  that  our  solar  system,  in  comparison, 
shrinks  to  insignificance.  But  far  more  wonderful  is  the 
cloud-like  mass  of  greenish-white  light  enveloping  these  stars. 
Just  visible  to  the  naked  eye  as  a  hazy  patch,  the  brighter  part 
is  readily  seen  with  a  pair  of  opera  glasses ;  but  to  get  an 
adequate  idea  of  its  beauty,  its  extent,  and  the  bewildering 
complexity  of  its  details  it  is  necessary  to  view  it  with  a  power- 
ful telescope,  or  to  study  a  photograph  taken  with  a  large 
modern  reflector.  It  is  hopeless  to  attempt  description,  just 
as  many  able  astronomers  have  found  it  hopeless  to  try  to 
portray  all  of  its  features  by  even  the  most  careful  drawings. 

I  have  called  it  a  nebula,  but  that  term  is  applied  to  at  least 
three  different  classes  of  objects,  the  spirals,  the  planetaries, 
and  the  irregular  gaseous  nebulae.  Our  object  belongs  to  the 
third  category,  for  the  spectroscope  long  ago  showed  that  it 


PLATE  XLII.    VACANT  LANES  AND  NEBULA  IN  TAURUS. 


Photographed  by  E.  E.  Barnard  with  the  10-inch  Bruce  telescope,  Jan.  9, 
1907,  Sl/2  hours'  exposure. 


NEWS    FROM    THE    STARS  161 

consists  of  gases  shining  by  inherent  light,  but  whether  this 
light  is  due  to  intense  heat  or  to  some  other  cause  it  has  until 
recently  been  quite  impossible  to  say.  Even  now  we  are  not 
prepared  to  assert  that  the  question  has  been  definitely  settled. 
The  great  difficulty  about  believing  it  to  be  due  to  heat  is  the 
almost  incredible  extent  and  tenuity  of  the  nebula.  On  the 
photographs  taken  with  the  Crossley  reflector  both  the  north 
and  south,  and  the  east  and  west  diameters  exceed  40  minutes 
of  arc.  To  translate  this  value  into  linear  measure,  miles 
or  kilometers,  it  is  necessary  to  know  how  far  away  the  object 
is.  This  we  do  not  know,  but  it  is  possible  to  set  a  minimum 
value  for  the  distance.  The  parallax  is  certainly  less  than  0.01 
second  of  arc;  that  is,  a  line  93,000,000  miles  long  (the  dis- 
tance from  the  Earth  to  the  Sun),  drawn  upon  the  surface  of 
the  nebula  would  to  our  eyes  subtend  an  angle  less  than  a 
hundredth  of  a  second  of  arc.  The  diameters  of  the  nebula 
are  therefore  certainly  more  than  240,000  (40X60X100)  times 
93,000,000  (22,320,000,000,000)  miles  and  may  be  more  than 
ten  times  as  great.  Some  one  has  computed  that  if  the 
material  were  only  1/1,000,000  as  dense  as  ordinary  atmos- 
pheric air  at  sea-level,  the  mass  of  the  nebula  would  be  so 
great  as  to  compel  all  of  the  stars  in  that  region  of  space  to 
travel  toward  it.  As  a  matter  of  fact  no  such  motion  is 
observed  and  the  tenuity  must  be  even  less  than  the  almost 
incredible  limit  named.  That  such  a  mass  of  matter  can  be 
hot  enough  to  be  incandescent  is  hard  to  believe,  but  recent 
investigations  indicate  that  this  is  the  case. 

Every  effort  has  been  made  to  determine  whether  there 
are  any  changes  in  the  position  of  the  nebula  as  a  whole  or 
in  any  of  its  parts,  but  without  positive  results.  This  does  not, 
of  course,  mean  that  the  nebula  is  absolutely  stationary  in 
space  but  only  that  whatever  motion  there  may  be  across  the 
line  of  sight  is  too  small  to  become  apparent  to  us  in  the  time 
during  which  accurate  measures  have  been  made.  In  this 
interval  there  may  have  been,  for  all  that  we  can  say,  a  motion 
of  translation  amounting  to  some  hundreds  of  millions  of  miles, 
but,  if  so,  the  resulting  angular  displacement  has  been  so  small 
that  we  have  not  been  able  to  detect  it.  The  spectroscope, 
however,  permits  us  to  make  accurate  measures  of  the  motion 


162  THE  ADOLFO  STAHL  LECTURES 

of  a  celestial  body  in  the  line  of  sight  no  matter  how  far  away 
the  body  may  be.  Its  testimony  is  to  the  effect  that  the  Sun 
and  the  nebula  as  a  whole  are  moving  away  from  each  other 
with  a  velocity  of  about  18  kilometers  a  second ;  but  by  far  the 
greater  part  of  this  relative  velocity  is  due  to  the  Sun's  own 
motion  through  space  and  only  a  small  fraction  to  the  actual 
motion  of  the  nebula.  In  fact,  this  nebula,  like  other  diffuse 
gaseous  nebulae,  seems  to  be  almost  stationary  when  compared 
to  the  motion  of  the  average  star. 

The  materials  of  the  nebula,  however,  are  far  from  being 
in  a  quiescent  state.  Three  or  four  years  ago  MM.  Buison, 
Fabry  and  Bourget,  at  Marseilles,  applied  an  interferometer 
attached  to  a  24-inch  reflecting  telescope  to  its  study.  In  effect 
this  apparatus  resembled  a  spectrograph  in  that  it  permitted 
the  observers  to  make  accurate  measures  of  the  radial  velocity 
of  the  portion  of  the  nebula  examined,  and  it  had  the  advantage 
over  the  ordinary  slit-spectrograph  of  permitting  such  measures 
to  be  made  over  every  part  of  a  field  some  four  minutes  of  arc 
in  diameter  on  a  single  photograph.  These  investigators  found 
that  different  parts  of  the  nebula  were  moving  with  different 
velocities.  The  interferometer  has  the  further  advantage  of 
giving,  under  certain  conditions,  a  theoretical  value  of  the 
atomic  weight  and  of  the  temperature  of  the  gas  whose  radia- 
tion is  measured  and,  in  the  present  instance,  the  authors  were 
led  to  conclude  that  the  atomic  weights  of  the  unknown  gases 
in  the  nebula  were  intermediate  between  that  of  hydrogen  and 
that  of  helium,  and  that  the  temperature  might  be  as  high  as 
15,000°  Centigrade.  Conclusions  of  such  fundamental  impor- 
tance to  our  theories  of  stellar  evolution  will,  of  course,  be 
most  carefully  verified  before  they  are  finally  adopted,  but  the 
ability  of  the  investigators  and  the  scrupulous  care  they  took 
to  check  their  work  at  every  stage  lend  great  weight  to  their 
results. 

Recent  spectrographic  measures  at  the  Lick  Observatory 
and  elsewhere  have  fully  confirmed  these  results  so  far  as  the 
internal  motions  are  concerned.  A  detailed  study  of  the 
Orion  Nebula  has  formed  part  of  the  program  of  work  with 
the  Mills  Spectrograph  during  the  past  few  years  and  accurate 
measures  of  the  radial  velocities  of  the  gases  in  many  different 


NEWS    FROM    THE    STARS  163 

parts  have  been  made.  It  is  found  that  in  some  parts  they  are 
receding  relatively,  in  other  parts  approaching,  the  relative 
velocities  occasionally  exceeding  10  kilometers  per  second. 
The  whole  mass,  therefore,  must  be  conceived  of  as  being  in 
seething  and  well-nigh  chaotic  turmoil. 

Now  this  is  one  of  the  latest  items  of  news  we  have 
received  from  the  Great  Nebula  in  Orion  and  it  illustrates  very 
well  the  impossibility  of  having  our  astronomical  news  even 
approximately  contemporaneous  with  the  event.  The  motions 
which  were  recorded  by  the  spectrograph  were  those  indicated 
by  the  light  waves  which  entered  the  slit,  but  those  light  waves 
left  the  nebula  certainly  more  than  300  years  ago ! 

Let  me  give  you  another  illustration.  Somewhat  east  of 
the  region  we  are  considering  there  is  a  star  known  as  Epsilon 
of  the  constellation  Hydra.  Long  ago  Struve  found  that  this 
was  a  double  star,  one  component  being  decidedly  fainter  than 
the  other.  In  1888,  Schiaparelli  noted  that  the  brighter  com- 
ponent was  itself  a  very  close  double,  the  two  components 
again  being  quite  unequal  in  brightness.  Now  I  followed  the 
motions  in  this  close  pair  by  measuring  the  relative  positions 
of  the  two  components  with  the  36-inch  telescope  for  15  years, 
during  which  time  the  fainter  star  seemed  to  make  a  complete 
revolution  about  the  brighter  one,  and  from  these  measures  I 
computed  the  elements  of  the  orbit.  At  the  same  time  measures 
made  with  the  spectrograph  showed  that  the  motion  of  the 
brighter  star  in  the  line  of  sight  was  variable  and  an  inde- 
pendent determination  of  some  of  the  elements  of  the  orbit 
was  thus  made  possible.  Moreover,  from  the  two  determina- 
tions it  was  possible  to  calculate  with  considerable  accuracy 
how  far  away  the  system  was.  It  proved  to  be  about  135  light- 
years  distant.  Therefore  the  revolution  of  the  two  stars  which 
I  witnessed  was  not  the  one  actually  taking  place  during  those 
15  years,  but  the  one  which  took  place  135  years  earlier,  or  dur- 
ing the  days  of  our  own  Revolutionary  War !  Since  then  the 
small  star  has  traveled  about  the  brighter  one  (more  precisely, 
the  two  stars  have  traveled  in  their  orbits  about  their  common 
center  of  gravity)  fully  nine  times  and  during  the  next  135 
years  the  light  waves  telling  us  of  those  motions  will  reach  the 
Earth.  It  is  literally  true  that  the  student  of  stellar  motions 


164  THE  ADOLFO  STAHL  LECTURES 

is  a  student  of  ancient  history  and  is  an  eye-witness  of  events 
which  happened  centuries  ago. 

Let  us  return  to  the  constellation  of  Orion.  The  drawing 
and  photograph  reproduced  on  Plate  XLIII  show  that  the  so- 
called  Great  Nebula  is  really  only  a  very  small  part  of  the  nebu- 
losity which  winds  about  the  entire  constellation.  This  vast 
faint  nebulosity  is  best  photographed  with  quite  small  tele- 
scopes, which  at  first  thought  may  seem  very  strange.  The  ex- 
planation is  found  chiefly  in  the  fact  that  our  large  telescopes 
cover  only  a  small  sky  area  at  any  one  time,  whereas  a  small 
telescope  of  reasonably  short  focal  length  includes  a  large  area. 
A  small  portion  of  the  outer  Orion  nebulosity  was  seen  by  Sir 
William  Herschel  with  his  great  reflector  more  than  a  century 
ago,  but  in  more  recent  years  its  existence  was  doubted  because 
certain  photographic  telescopes,  of  great  power  for  many 
classes  of  work,  failed  to  show  it.  In  1889,  however,  Professor 
W.  H.  Pickering,  in  the  course  of  his  tests  of  atmospheric 
conditions  on  Mount  Wilson,  now  the  site  of  the  Solar 
Observatory,  photographed  this  remarkable  object  with  a 
portrait  lens  of  2.6  inches  aperture  and  8.6  inches  equivalent 
focus.  In  1894,  Professor  Barnard  was  experimenting  at  the 
Lick  Observatory  with  a  little  lens  taken  from  a  cheap  (oil) 
projecting  lantern.  The  lens  was  but  1.6  inches  in  diameter 
and  had  an  equivalent  focus  of  6.3  inches.  Unaware  of 
Pickering's  work,  he  photographed  the  constellation  of  Orion 
and  fully  verified  the  existence  of  this  great  enveloping  nebula. 
In  gathering  news  from  the  stars,  then,  we  use  visual  and 
photographic  telescopes  ranging  in  aperture  from  a  single  inch 
to  the  100  inches  of  the  great  reflector  on  Mount  Wilson,  and 
we  attach  to  these  our  spectrographs,  photometers  and  other 
apparatus  for  special  investigations. 

There  are  other  constellations  which  contain  similar 
diffused  and  faint  nebulae.  One,  the  ihost  interesting  of  these, 
surrounds  the  little  group  of  the  Pleiades,  in  the  constella- 
tion Taurus,  a  group  of  stars  that  is,  perhaps,  the  most  familiar 
of  any  in  the  sky.  The  average  eye  sees  six  stars  in  this  little 
group ;  keener  eyes,  especially  in  the  clear  air  of  mountain 
regions,  distinguish  seven  or  eight  or  even  more.  A  small 
telescope  greatly  increases  the  number,  but,  unlike  some  glob- 


J 


.    •  i 

*        o 

•r 

•r  ' 


A.     Drawing  from  two  lantern-lens  photographs    (Oct.   3  and  24,   1894) 


B.     Photograph,  with  Willard  lens,  of  region  enclosed  in  the  square  in 
the  drawing  above  (Oct.  17,  1893),  3  hours'  exposure. 


PLATE  XLIII.    THE  GREAT  CURVED  NEBULA  IN  ORION,  BY  E.  E.  BARNARD. 


NEWS    FROM    THE    STARS  165 

ular  clusters,  the  number  can  not  be  increased  indefinitely  by 
photographing  the  region  with  telescopes  of  ever  greater 
power.  The  entire  group,  as  I  have  said,  is  involved  in  nebu- 
losity similar  to  that  which  encircles  Orion.  This  was  first 
noted  by  Professor  Barnard  but  has  since  been  photographed 
by  a  number  of  different  astronomers.  Attention  in  recent 
years  has  been  concentrated  upon  other  features  of  the 
Pleiades  group,  particularly  upon  the  brighter  stars  and  upon 
certain  remarkable  nebulae  associated  with  them. 

The  most  recent  study  of  the  stars  in  the  cluster  is  that 
just  completed  by  Dr.  Trumpler,  at  the  Allegheny  Observatory. 
He  finds  that  the  cluster  includes  from  80  to  90  stars  as  bright 
as  9.0  magnitude  (bright  enough,  that  is,  to  be  just  visible  in  a 
telescope  of  one-inch  aperture),  with  probably  55  more  stars 
between  magnitudes  9.0  and  9.5.  Doubtless,  stars  still  fainter 
belong  to  the  cluster  but  a  large  percentage  of  the  faint  stars 
of  the  region  certainly  form  part  of  the  stellar  background 
upon  which  we  see  the  cluster  projected.  We  can  distinguish 
between  the  two  classes  of  stars  by  the  fact  that  the  cluster 
stars  are  moving  together  through  space.  And  it  also  appears 
that  the  stars  thus  moving  together  resemble  each  other  in  the 
character  of  their  spectra.  These  two  qualities,  community 
of  motion  and  resemblance  of  spectrum,  lead  us  to  conclude 
that  the  stars  in  the  cluster  had  a  common  nebulous  origin,  and 
it  is  not  at  all  improbable  that  in  the  nebulosity  surrounding 
the  group  we  see  the  remnant  of  the  material  out  of  which  the 
stars  were  formed. 

In  addition  to  the  apparent  association  of  the  stars  and  neb- 
ulosity, there  are  several  arguments  in  favor  of  this  view.  For 
example,  long-exposure  photographs,  like  those  taken  with  the 
Crossley  reflector,  show  that  some  of  the  brightest  stars  in  the 
group  are  immersed  in  nebulosity  and  the  spectrograph  testifies 
that  they  have  extensive  gaseous  atmospheres  with  relatively 
small  cores  of  denser  matter.  In  other  words,  they  are  prob- 
ably still  in  the  earliest  stages  of  their  development  as  stars. 
Again,  Slipher  has  shown  that  the  light  of  the  nebula  associated 
with  Merope,  one  of  the  bright  stars  of  the  Pleiades,  has  pre- 
cisely the  same  quality  as  the  light  of  the  star.  He  finds  the 
same  to  be  true  of  the  star  Maia  and  its  nebula,  and,  more 


166  THE  ADOLFO  STAHL  LECTURES 

recently,  he,  and  Pease  at  the  Solar  Observatory,  have  found 
two  other  instances  of  star  and  nebula  which  possess  light  of 
identical  quality.  Slipher  has  argued  that  this  indicates  that 
the  nebula  is  shining  not  by  its  inherent  light  but  by  light 
reflected  from  the  star  or  stars,  and  Hertzsprung's  photometric 
measures  in  the  case  of  the  Merope  nebula  bring  confirmatory 
evidence.  Whether  we  accept  or  reject  the  explanation,  the 
observations  show  the  close  connection  of  the  two  classes  of 
objects. 

It  is  a  most  interesting  fact  that  Merope  and  Maia  and  the 
other  two  stars  which  have  so  far  been  found  to  be  attended  by 
nebulae  radiating  light  of  the  same  quality,  are  "helium  stars," 
that  is,  stars  in  whose  spectrum  the  lines  of  helium  are  strongly 
marked.  For  the  stars  in  general  have  been  classified  according 
to  the  character  of  the  spectrum  they  exhibit  and  it  has  been 
found  that  the  blue-white  helium  stars  stand  at  one  end  of  a 
continuous  series  running  through  white,  yellow,  orange  and 
red  to  deep  red  stars.  On  what  may  be  called  the  classical 
theory  of  stellar  evolution  this  order  represents  the  successive 
stages  of  stellar  development  from  infancy  to  old  age.  In  recent 
years  the  classical  theory  has  been  strongly  challenged  and  a 
substitute  theory  offered  according  to  which  the  youngest  stars 
as  well  as  the  oldest  are  red  and  the  blue-white  stars  occupy  a 
middle  position.  This  is  not  the  place  to  present  the  forceful 
arguments  brought  to  the  support  of  each  of  these  hypotheses, 
or  to  discuss  their  relative  merits.  I  have  mentioned  them 
merely  to  point  out  that  one  of  the  greatest  difficulties  in  the 
way  of  the  acceptance  of  the  two-branched  evolutionary  theory 
is  the  close  association  of  the  helium  stars  with  diffuse  nebu- 
losity such  as  exists  in  the  constellation  of  Orion  and  in  the 
Pleiades.  There  is  no  correlation  whatever  between  such 
nebulae  and  red  stars. 

This  is  perhaps  as  good  a  place  as  any  to  insist  upon  the 
necessity  of  discriminating  between  the  facts  of  observation 
and  the  theories  which  may  be  based  upon  those  facts. 
Though  elementary,  the  distinction  is  frequently  lost  sight  of 
and  astronomy,  or  rather  the  reputation  of  the  astronomer, 
suffers.  It  is  a  fact  that  the  companion  star  in  the  system  of 
Epsilon  Hydrae  changes  its  position  continuously  with  respect 


PLATE  XLIV.     THE  PLEIADES. 


From  a  photograph  by  Sir  Isaac  Roberts,  Dec.  8,  1888,  exposure  4  hours. 


NEWS    FROM    THE    STARS  167 

to  the  brighter  star  in  such  a  manner  that  after  15  years  it 
returns  to  its  apparent  starting  point.  The  theory  is  that  the 
change  is  due  to  the  motion  of  the  two  bodies  in  elliptic  orbits 
about  a  common  center  of  gravity  under  the  law  of  gravitation. 
In  this  case  the  evidence  from  numerous  double  stars  is  so 
overwhelmingly  strong  that  the  theory  has  as  much  weight  as 
the  observed  facts.  In  other  instances,  as  for  example,  the 
identity  in  the  quality  of  the  light  of  star  and  nebula  or  the 
arrangement  of  stellar  spectra,  the  facts  are  beyond  question 
but  they  may  perhaps  be  subject  to  more  than  one  interpreta- 
tion. We  are  quite  willing  to  abandon  any  theory,  however 
cherished,  whenever  the  facts  fail  to  support  it. 

Let  us  again  return  to  the  constellation  of  Orion  in  order 
to  consider  a  photograph  taken  by  Dr.  Curtis  with  the  Cross- 
ley  reflector  only  a  week  ago  (Plate  XLV).  The  photograph 
shows  the  region  just  south  of  Zcta  Orionis,  the  eastern  star 
of  the  three  in  the  "Belt."  Passing  over  other  features,  I  want 
to  call  your  attention  to  the  sharply  marked  dark  blotch,  like 
an  ink-blot,  just  above  the  center  of  the  picture.  At  first  sight 
it  might  be  taken  for  a  defect  of  some  kind  in  the  film.  That 
it  is  not  a  defect  was  demonstrated  by  the  fact  that  it  reap- 
peared in  identically  the  same  position  on  a  different  plate  of 
the  region  taken  on  the  following  night.  The  reality  of  the 
marking  being  thus  established,  the  question  arises  whether  it 
represents  a  non-luminous  substance  which  obstructs  the 
passage  of  light  from  the  luminous  area  into  which  it  projects 
or  whether  it  is  a  vacant  region  in  space,  a  "tunnel"  bored 
through  the  fabric  of  the  constellation.  This  particular  mark- 
ing has  not,  so  far  as  I  am  aware,  been  photographed  before ; 
the  picture  before  you  gives  one  of  the  latest  items  of  news 
received  from  the  stars;  but  "black  holes"  have  long  been 
known  in  certain  regions  of  the  Milky  Way  and  are  beautifully 
pictured  in  many  of  Barnard's  photographs  as  well  as  in  those 
taken  by  other  observers. 

Twenty  years  ago  it  was  thought  not  impossible  that  these 
markings  might  really  represent  vacant  regions  of  space,  but 
further  investigation  of  them  with  modern  photographic  tele- 
scopes, an  investigation  in  which  Professor  Barnard  has  been 
especially  prominent,  has  led  to  the  abandonment  of  this 


168  THE  ADOLFO  STAHL  LECTURES 

hypothesis  by  most  astronomers.  The  edges  of  the  markings 
are  usually  far  too  sharp,  the  forms  are  frequently  too  strik- 
ingly similar  to  those  of  bright  nebulae,  and  too  many  of  the 
dark  patches  are  found  in  regions  where  it  is  impossible,  on 
any  reasonable  theory  of  stellar  distribution,  to  account  for 
the  sudden  absence  of  faint  stars. 

It  is  of  course  conceivable  that  a  compact  cluster  of  stars 
rushing  through  space  might  clear  a  path  for  itself  and  leave 
a  vacant  lane ;  but  in  the  cases  known  to  us  there  is  no  evidence 
of  the  existence  of  such  a  cluster  in  any  position  where  it 
might  be  assumed  to  be  after  making  such  a  lane.  Hence  if 
any  one  of  these  "holes"  had  such  an  origin  the  cluster  must 
have  passed  many  million  years  ago.  But  in  this  event  we 
would  not  have  the  sharply  cut  outlines,  for  the  stars  are  all  in 
motion  and,  as  Dr.  Campbell  has  pointed  out,  these  motions 
would  in  such  a  time  interval  have  carried  many  stars  into  the 
vacant  region,  obliterating  the  clear-cut  edges  and  possibly 
the  "hole"  itself. 

On  the  other  hand,  Bessel  long  ago  remarked  that  lumi- 
nosity is  not  a  necessary  property  of  cosmical  bodies.  "The 
visibility  of  countless  stars  is  no  argument  against  the  invisi- 
bility of  countless  others."  If  this  may  be  true  of  stars  there  is 
no  apparent  reason  why  it  may  not  be  true  also  of  nebulae.  As 
a  matter  of  fact  we  have  quite  definite  evidence  of  the  existence 
of  dark  objects  of  both  classes  and  some  of  the  strongest  of 
this  evidence  is  furnished  by  the  novae,  or  new  stars,  such  as 
are  the  subject  of  the  newspaper  article  to  which  I  referred 
a  little  while  ago.  By  a  nova  is  meant  a  star  which  suddenly 
appears  in  a  spot  where  no  star  was  previously  known  to 
exist.3  In  recent  years  a  number  of  such  new  stars  have  been 
discovered,  especially  by  photography,  and  in  every  case  they 
have  exhibited  quite  similar  phenomena.  The  brightness 
increases  enormously  in  a  very  short  period  of  time ;  maximum 

3  In  a  few  instances  a  nova  has  been  identified  with  a  very  faint  star  known 
for  years  before  its  sudden  outburst.  Perhaps  the  best  example  is  the  brilliant 
nova  which  appeared  in  the  constellation  Aquila  on  June  8,  1918,  and  on  the 
following  night  rivaled  Sirius  in  brightness.  The  astronomers  at  the  Harvard 
College  Observatory  photograph  the  entire  sky  on  a  systematic  plan  many  times 
each  year,  and  the  plates  thus  secured  form  an  invaluable  photographic  reference 
library.  An  examination  of  the  appropriate  plates  enabled  Professor  E.  C.  Picker- 
ing to  state  at  once  that  the  nova  had  been  visible  as  a  faint  star  (llth  magni- 
tude) at  least  as  long  ago  as  May  22,  1888,  for  it  was  photographed  On  that  date. 


FIG.  1— In  Orion   (5h  36.0m;— 2°  27'). 


FIG.  2— In  Sagittarius  (17h  56.6m;— 27°  50'). 

PLATE  XLV.     DARK  NEBULAE. 
Photographs  by  H.  D.  Curtis,  Crossley  Reflector. 


NEWS    FROM    THE    STARS  169 

brightness  lasts  for  a  few  days  or  hours  only  and  is  followed 
by  a  more  or  less  gradual  decline  which  often  proceeds  to  the 
point  of  absolute  invisibility ;  and  the  various  stages  of  its 
light  curve  are  synchronal  with  well  defined  changes  in  the 
spectrum.  Various  explanations  of  these  phenomena  have 
been  offered.  Certainly  a  nova  is  the  result  of  a  celestial 
catastrophe  of  some  kind,  but  no  completely  satisfactory 
explanation  of  the  nature  of  the  catastrophe  has  so  far  been 
found.  The  most  plausible  theory  (though  one  not  entirely 
free  from  objections)  is  that  the  outer  strata  of  a  dark  or 
nearly  dark  star  rushing  through  a  region  of  space  filled  with 
more  or  less  dense  nebulosity  are  heated  to  incandescence,  the 
depth  of  the  incandescent  strata  and  the  intensity  of  the  con- 
sequent luminosity  depending  upon  the  degree  of  resistance 
encountered  by  the  star.  In  at  least  one  instance,  Nova  Persei 
of  1901,  we  know  that  the  new  star  was  attended  by  nebu- 
losity which,  in  appearance,  was  expanding  in  all  directions 
from  the  star.  Now  this  nebulosity  was  not  known  before  the 
star's  outburst.  Possibly  it  was  entirely  dark,  like  the  nebula 
south  of  Zeta  Orionis,  but  not  dense  enough  to  manifest  itself 
by  contrast,  as  the  latter  does ;  possibly  it  was  feebly  luminous 
and  might  have  been  detected  had  the  region  been  photo- 
graphed with  a  suitable  telescope  and  a  sufficiently  long 
exposure.  But  though  unknown  we  are  reasonably  certain 
that  it  existed  independently  of  the  star  and  was  not  a  product 
of  the  latter's  outburst,  for  the  observed  angular  velocity,  when 
converted  into  miles  per  second  on  the  basis  of  the  minimum 
possible  distance  separating  us  from  the  star,  was  so  enormous 
that  we  cannot  believe  we  were  witnessing  the  actual  transla- 
tion of  material  particles.  Far  more  reasonable  is  the  hypoth- 
esis, first  suggested  by  Kapteyn,  that  the  apparent  motion 
was  due  to  the  great  wave  of  light  sent  out  from  the  star.  As 
this  wave  reached  successive  portions  of  the  nebula  these 

Several  hundred  plates  of  the  region  taken  on  later  dates  show  a  relatively 
slight  variation  in  its  brightness  (about  half  a  magnitude),  but  it  was  still 
approximately  of  the  llth  magnitude  on  June  3,  1918.  Clouds  prevented  photo- 
graphs on  the  next  three  nights,  but  on  a  plate  taken  on  June  7th  the  star  was 
very  much  brighter,  being  of  the  6th  (photographic)  magnitude.  Light  waves 
from  the  sudden  great  outburst,  then,  began  to  reach  the  Earth  sometime  between 
the  3d  and  6th  of  June,  so  that  we  know  positively  that  less  than  six  days  were 
required  for  a  100,000-fold  increase  in  the  star's  brilliancy  {\2l/2  stellar  magni- 
tudes). This  is  the  brightest  nova  known  since  Kepler's  star  in  Ophiuchus  which 
appeared  in  1604. 


170  THE  ADOLFO  STAHL  LECTURES 

became  visible  to  us,  shining  by  reflected  starlight  as  the  Moon 
shines  by  reflected  sunlight.  After  the  wave  passed,  each 
part  in  succession  again  became  invisible  and  the  effect  was 
that  of  nebular  material  moving  radially  from  the  star  with 
the  velocity  of  light.  Nova  Persei  increased  in  light  fully 
60,000- fold  (12  magnitudes)  in  less  than  five  days,  and  quite 
rapidly  lost  a  large  portion  of  its  light  after  reaching  its  maxi- 
mum ;  and  calculation  has  shown  that  when  it  was  at  maximum 
brightness  its  light  was  intense  enough  to  affect  our  photo- 
graphic plates  if,  reflected  from  nebulous  matter  at  the  dis- 
tances where  this  was  actually  observed.  Slipher's  recent 
work,  to  which  I  have  already  referred,  affords  strong  col- 
lateral evidence  in  support  of  this  theory,  inasmuch  as  it  gives 
us  instances  of  other  nebulae  which  are  quite  probably  shining 
in  whole  or  in  part  by  light  reflected  from  the  stars  with  which 
they  are  associated. 

The  novae  are  exceedingly  interesting  objects  and  might 
well  be  made  the  subject  of  an  independent  lecture.  Here  I 
can  only  take  time  to  tell  you  one  or  two  of  the  latest  news 
items  we  have  regarding  them.  Up  to  July,  1917,  32  novae 
had  become  known,  the  majority  of  them  in  comparatively 
recent  years  and  largely  through  the  comparison  of  photo- 
graphic plates.  With  but  three  exceptions  all  of  these  new 
stars  were  situated  in  the  Milky  Way;  of  the  exceptional 
cases  one  (T  Coronae)  was  not  a  typical  nova  and  the  other 
two  appeared  in  spiral  nebulae.  Since  July,  seventeen  addi- 
tional novae  have  been  announced,  fifteen  of  them  in  spiral 
nebulae.41 

Now  this  distribution  is  a  very  remarkable  one,  especially 
when  we  recall  the  fact  that  several  different  lines  of  investiga- 
tion are  leading  astronomers  to  regard  with  increasing  favor 
the  theory  that  the  spirals  are  not  members  of  our  own  stellar 
system  but  are  independent  systems,  "island  universes."  That 
the  new  stars  are  actually  in  the  spirals  and  not  between  us 

4  The  figures  for  the  number  of  novae  have  been  changed  in  this  paragraph  and 
those  following  to  correspond  to  the  state  of  our  knowledge  early  in  December, 
1918.  Two  novae — Nova  Monocerotis  and  Nova  Aquila  No.  3  (see  footnote  3) — 
have  been  discovered  in  the  Milky  Way  since  the  lecture  was  delivered,  and  seven 
in  spiral  nebulae.  It  is  a  remarkable  fact  that  eleven  of  the  seventeen  novae  now 
known  to  have  appeared  in  spiral  nebulae  have  been  found  in  the  Great  Nebula  of 
Andromeda,  eight  of  them  appearing  in  the  short  interval  between  July,  1917,  and 
November,  1918. 


NEWS    FROM    THE    STARS  171 

and  the  nebulae  is  beyond  question.  A  single  nova  might 
perhaps  appear  projected  upon  a  nebula  to  which  it  did  not 
belong ;  that  seventeen  should  appear  in  the  line  of  sight  toward 
some  spiral  nebula  is,  as  Curtis  has  remarked,  "manifestly 
beyond  the  bounds  of  probability". 

These  recent  discoveries  are  one  result  of  the  intensive 
study  of  the  spirals  which  has  been  in  progress  at  several 
different  observatories  in  the  last  few  years.  It  was  Ritchey, 
at  the  Solar  Observatory,  who  found  the  first  one.  A  photo- 
graph of  the  spiral  known  as  N.  G.  C.  6946,*  which  he  secured 
on  July  19,  1917,  with  the  60-inch  reflector,  showed  a  star  of 
14.6  magnitude  that  did  not  exist  on  plates  of  the  nebula  taken 
in  1910,  1912,  1915  and  1916,  some  of  which  showed  stars  as 
faint  as  the  21st  magnitude.  By  August  16th  the  star  had  lost 
more  than  half  of  its  light  and  may  now  be  once  more  too  faint 
to  be  photographed. 

Ritchey's  discovery  at  once  set  astronomers  at  work  com- 
paring all  available  photographs  of  spirals.  Curtis  at  the  Lick 
Observatory  promptly  added  three  more  novae  by  his  study  of 
the  Crossley  reflector  plates,  and  Ritchey,  Pease,  Shapley, 
Duncan  and  Sanford  at  the  Solar  Observatory  have  increased 
the  number  to  fifteen.  The  apparition  period  of  a  nova  may 
be  limited  to  a  few  months  or  even  to  a  few  weeks  and  it  is 
easy  to  see  that  many  novae  may  have  appeared  in4  spirals  at 
times  when  no  photographs  were  taken.  Now  that  attention 
has  been  directed  to  their  relatively  frequent  occurrence  in 
these  objects  .we  may  expect  the  number  of  such  discoveries 
to  increase  more  rapidly. 

It  is  interesting  to  note  that  the  new  series  of  novae  are  all 
very  faint  objects  as  seen  from  the  Earth.  On  the  average 
they  have  only  reached  the  14th  magnitude  at  maximum  bright- 
ness and  at  minimum  light  have  certainly  fallen  below  the 
21st  magnitude  in  nearly  every  instance.  The  Milky  Way 
novae  discovered  during  the  last  25  years  have  attained,  at 
maximum,  magnitudes  ranging  from  about  — I A  to  +11,  the 
average  being  about  +6,  or  eight  magnitudes  brighter  than  the 
average  for  the  novae  in  spirals. 

*  From  its  number  in  Dreyer's  New  General  Catalogue  of  Nebulae  and 
Clusters  of  Stars. 


172  THE  ADOLFO  STAHL  LECTURES 

Let  us  assume  that,  on  the  average,  the  new  stars  in  the 
two  sets  attain  the  same  absolute  luminosity  at  maximum  light ; 
then  it  follows  that,  on  the  average,  the  novae  in  spirals  are 
at  least  40  times  as  distant  as  those  in  our  Milky  Way.  If  we 
take  20,000  light-years  as  the  probable  distance  for  the  latter, 
the  former  are  800,000  light-years  distant.  It  is  obvious  from 
such  an  argument  that  the  discovery  of  novae  in  spirals  has  a 
definite  bearing  upon  the  theory  that  the  spirals  are  independent 
or  "island  universes".5  The  theory  may  not  be  correct,  the 
argument  may  be  fallacious;  but  it  is  just  such  hypotheses  and 
deductions  that  the  astronomer  must  make.  For  while  it  is  his 
first  duty,  like  that  of  the  reporter  for  the  daily  press,  to  gather 
the  facts  and  describe  them  accurately,  his  ultimate  purpose, 
since  he  is  a  scientific  investigator,  is  to  correlate  the  newly 
observed  facts  with  those  already  known  and  thus  finally 
discover  the  natural  laws  of  whose  operation  the  phenomena 
are  the  manifestation.  Even  a  false  hypothesis  may  help  him 
toward  the  truth  provided  he  preserves  an  open  mind  and  is 
willing  to  discard  it  or  to  modify  it  as  additional  facts  may 
require. 

It  has  been  my  endeavor  in  this  hour's  talk  to  bring  before 
you  a  very  few  of  the  latest  items  of  news  from  the  stars  and 
by  means  of  them  to  illustrate  the  nature  of  the  work  upon 
which  the  astronomer  is  engaged.  I  have  had  another  pur- 
pose also,  and  that  is  to  take  your  thoughts  away,  for  a  short 
time,  from  the  cares  and  anxieties  of  our  every-day  life.  It  is 
with  deliberate  purpose,  too,  that  I  have  included  in  my  items 
several  which  relate  to  the  stars  now  visible  in  our  early  eve- 
ning sky,  for  I  hope  that  you  may  be  led,  from  time  to  time,  to 
look  up  thoughtfully  at  these  stars.  If  you  will  do  so,  I  think 
you  will  find  that,  as  a  recent  English  writer  says,  "the  stars 
have  a  balm  for  us  if  we  will  but  be  silent,"  for  the  "huge  and 
thoughtful  night  speaks  a  language  simple,  august,  universal." 

"It  is  one  of  the  minor  consolations  of  the  war,"  continues 
this  writer,  who  is  personally  doing  his  utmost  to  support  his 
government  in  the  prosecution  of  the  war,  "that  it  has  given  us 
in  London  a  chance  of  hearing  that  language.  The  lamps  of 


5  See  also  the  notes  by  Curtis  and  by  Shapley  in  the  Publ.  Asiron.  Soc.   Pac., 
29,   180,   213,   1917. 


PLATE  XLVI. 


The  Great  Nebula  in  Andromeda  and  the  locations  of  ten  Novae  dis- 
covered at  the  Solar  Observatory.  Nova  No.  2  is  visible  on  the 
photograph. 


NEWS    FROM    THE    STARS  173 

the  streets  are  blotted  out,  and  the  lamps  above  are  visible — the 
great  procession  of  the  stars  is  the  most  astonishing  spectacle 
offered  to  men.  Emerson  said  that  if  we  only  saw  it  once  in  a 
hundred  years  we  should  spend  years  in  preparing  for  the 
vision.  It  is  hung  out  for  us  every  night,  and  we  barely  give 
it  a  glance.  And  yet  it  is  well  worth  glancing  at.  It  is  the  best 
corrective  for  this  agitated  little  mad-house  in  which  we  dwell 
and  quarrel  and  fight  and  die.  It  gives  us  a  new  scale  of 
measurement  and  a  new  order  of  ideas.  Even  the  war  seems 
only  a  local  affair  of  some  ill-governed  asylum  in  the  presence 
of  this  ordered  march  of  illimitable  worlds." 


RECENT  PROGRESS  IN  THE  STUDY  OF 
MOTIONS  OF  BODIES  IN  THE  SOLAR  SYSTEM1 

By  ARMIN  O.  LEUSCHNER 

INTRODUCTORY  REMARKS 

During  the  past  ten  days  astronomers  all  over  the  world 
have  been  startled  by  the  announcement  of  the  discovery  of  a 
mysterious  object  of  starlike  appearance  moving  over  a  degree 
a  day  in  a  northeasterly  direction.  If  the  discovery  represents 
a  minor  planet,  of  which  nearly  one  thousand  are  known  at 
the  present  time,  the  object  would  nevertheless  be  of  great 
importance  astronomically  because  its  observed  angular  motion 
around  the  Sun  exceeds  that  of  the  Earth. 

According  to  Kepler's  Harmonic  Law  the  angular  motion 
of  bodies  moving  around  the  Sun  diminishes  more  rapidly 
than  the  distance  increases.  Minor  planets  are  generally 
discovered  when  the  Earth  is  somewhere  between  them  and 
the  Sun,  so  that  the  distance  of  the  planet  from  the  Sun  is 
greater  than  the  distance  from  the  Earth.  As  seen  from  the 
Earth,  the  planet  would  then  in  general  appear  to  move  in  the 
opposite  direction,  or  westward.  This  motion,  known  as 
retrograde  motion,  is  the  usual  motion  of  planets  when  dis- 
covered near  opposition. 

The  fact  that  this  object,  as  seen  from  the  Earth,  is  moving 
in  an  easterly  direction  nearly  a  degree  a  day  indicates  that 
its  angular  motion  around  the  Sun  is  greater  than  that  of  the 
Earth.  This  apparently  contradicts  the  Harmonic  Law  of 
Kepler,  but  this  law  applies  merely  to  the  average  motion 
which  is  uniform  in  the  orbit  when  the  orbit  is  nearly  circular, 
but  if  the  orbit  is  very  eccentric  then  the  angular  motion  of  a 
minor  planet  near  perihelion  is  far  greater  than  near  aphelion. 
Hence  if  a  minor  planet  moving  in  a  very  eccentric  orbit  is 
discovered  near  perihelion  a  situation  such  as  is  presented  by 
the  mysterious  object  recently  discovered  might  exist. 

1  Delivered   February    15,    1918. 


MOTIONS  IN  THE  SOLAR  SYSTEM  175 

The  observed  position  and  motion  of  the  object  therefore  at 
once  lead  to  the  conclusion  that  it  is  moving  in  a  highly  eccen- 
tric orbit.  The  possibility  of  the  object  being  a  tiny  moon 
revolving  around  the  Earth  is  excluded  by  the  fact  that  its 
motion  then  would  have  to  be  approximately  in  a  great  circle 
of  the  celestial  sphere,  but  this  is  contradicted  by  the  observa- 
tions. The  choice  therefore  lies  between  a  minor  planet 
moving  in  an  unusually  eccentric  orbit  and  discovered  near 
perihelion,  and  a  comet  of  a  very  unusual  appearance.  Comets 
usually  move  in  highly  eccentric  ellipses,  but  even  if  they  have 
a  starlike  appearance,  the  larger  telescopes  readily  identify 
them  as  comets  by  the  nebulosity  which  surrounds  them.  The 
discoverer  evidently  was  not  able  to  commit  himself  as  to  the 
nature  of  the  object  and  it  has  therefore  been  announced  under 
the. general  designation  "Object  Wolf". 

It  is  not  the  first  time,  however,  that  a  comet  has  been 
discovered  with  a  distinctly  stellar  appearance.  In  1913  such 
a  comet  was  discovered  by  Neujmin  in  Russia,  and  was 
designated  for  some  time  as  an  "object"  until  the  discovery 
with  larger  telescopes  of  nebulosity  surrounding  it,  and  the 
calculation  of  the  orbit  definitely  placed  it  in  the  class 
of  comets. 

The  first  telegram  announcing  the  discovery  of  the  Wolf 
object  was  received  in  America  a  week  ago  (February  7,  1918). 
The  original  telegram  contained  two  approximate  photo- 
graphic positions  obtained  on  February  3  and  4.  On  Monday, 
February  11,  another  cable  was  received  giving  the  third  and 
accurate  position  of  the  object.  As  the  first  observations  are 
generally  secured  in  haste,  so  as  to  make  sure  that  sufficient 
data  shall  be  available  for  the  necessary  calculations,  errors  in 
observation  occur  frequently.  These  errors  naturally  are  a 
source  of  unnecessary  labor  and  annoyance  to  the  investigator 
of  the  orbit. 

Since  the  first  two  European  observations  were  only 
approximate,  giving  the  position  merely  to  the  nearest  minute 
of  arc,  some  hesitancy  was  felt  in  attacking  the  problem.  But 
the  second  telegram  contained  further  announcement  of  such  a 
character  that  the  object  became  even  of  greater  interest,  for 
it  stated  that  revolving  about  the  object  was  another  consider- 


176  THE  ADOLFO  STAHL  LECTURES 

ably  fainter  object  at  a  distance  of  340  seconds  of  arc  as  seen 
from  the  Earth,  and  revolving  about  it  at  the  rate  of  13  degrees 
an  hour,  so  that  the  complete  revolution  would  be  made  by  the 
satellite  in  about  36  hours.  In  spite  of  the  meager  data  an 
attempt  was  therefore  made  to  learn  something  about  the 
general  character  of  the  orbit.  The  computation  was  under- 
taken by  Professor  R.  T.  Crawford  and  Mr.  H.  M.  Jeffers. 
They  soon  found  that  the  problem  did  not  admit  of  a  solution. 
In  other  words,  no  object  could  exist  moving  in  the  manner 
in  which  the  observations  indicated.  The  only  other  alterna- 
tive would 'be  that  one  of  the  observations  was  seriously 
in  error. 

While  an  attempt  was  under  way  to  locate  a  possible  error, 
a  telegram  was  received  from  Director  Campbell  informing  us 
that  Dr.  H.  D.  Curtis  had  photographed  the  object  with  the 
Crossley  reflector,  and  had  accurately  measured  its  position. 
Tabulation  of  the  now  available  four  positions  (the  European 
positions  being  taken  on  February  3,  4  and  5,  and  the  Mount 
Hamilton  position  on  February  11)  enabled  us  to  suspect 
that  the  first  European  right  ascension  might  be  in  error.  Mr. 
Jeffers  made  a  new  solution  on  the  basis  of  numerical  data 
which  I  readily  estimated  from  the  observations.  Such  a 
solution  may  be  made  in  two  ways — either  with  or  without 
previous  assumption  regarding  the  nature  of  the  orbit.  Since 
comets  usually  move  in  highly  eccentric  orbits  which  are  not 
very  different  from  parabolas,  it  is  customary  to  assume  the 
orbit  to  be  parabolic.  Mr.  Jeffers  made  an  approximate  general 
solution  and  found  that  the  object  moved  in  a  pronounced 
hyperbola,  but  astronomical  experience  teaches  us  that  there  is 
no  object  in  existence  which  moves  in  a  pronounced  hyper- 
bola. Again  we  seemed  to  be  confronted  with  something 
impossible,  but  Mr.  Jeffers,  who  was  performing  the  calcula- 
tion, correctly  concluded  that  the  hyperbola  was  merely  a 
result  of  the  uncertainty  of  solution.  Such  uncertainty  may  be 
removed  by  making  a  conditioned  solution  assuming  the  orbit 
to  be  a  parabola.  The  resulting  orbit  fitted  all  four  available 
observations  with  the  exception  of  the  first  European  right 
ascension  which  had  been  suspected  to  be  in  error. 

An  American  astronomer  reported  that  on  a  photograph 


MOTIONS  IN  THE  SOLAR  SYSTEM  177 

the  object  appeared  to  be  surrounded  by  a  slight  nebulosity. 
The  approximate  orbit  and  the  reported  nebulosity  point  to 
the  possibility  that  the  object  may  be  a  comet.  But  there  still 
exists  the  possibility  that  the  parabolic  orbit  is  merely  an 
approximation  to  a  highly  eccentric  ellipse,  which  can  be 
derived  only  on  the  basis  of  more  accurate  observations  to  be 
secured  within  the  next  few  days. 

In  the  meantime  we  must  await  developments.  These 
recent  experiences  illustrate  the  first  stages  of  an  orbit 
investigation  and  serve  to  show  how  much  the  investigator  of 
the  orbit  depends  upon  accurate  observations  for  a  satisfactory 
result  of  his  work.2 

LECTURE3 

The  study  of  motions  of  bodies  of  the  solar  system,  like  the 
object  just  referred  to,  is  based  on  Newton's  law  of  gravita- 
tion. This  law  states  that  every  particle  of  matter  in  the 
universe  attracts  every  other  particle  of  matter  with  a  force 
which  is  proportional  to  the  product  of  their  masses,  and 
inversely  proportional  to  the  square  of  their  distance.  This 
means  that  if  two  bodies,  each  as  massive  as  the  Sun,  and  at 
a  distance  apart  which  equals  the  distance  of  the  Earth  from 
the  Sun,  attract  each  other  with  a  certain  force,  then  two 
similar  bodies  at  twice  the  distance  attract  each  other  with 
one-fourth  the  force;  at  three  times  the  distance  with  one- 
ninth  the  force.  Further,  if  one  of  the  bodies  be  replaced  by 
a  body  one-half  as  massive  as  the  Sun,  then  the  force  is  one- 
half ;  if  one  body  is  one-third  as  massive  and  the  other  one- 
fourth,  then  the  force  will  be  one-twelfth.  This  law  of  gravi- 
tation, together  with  three  axioms  or  laws  of  motion  an- 
nounced by  Newton  in  1686,  which  we  need  not  consider  here, 
have  served  for  the  interpretation  of  the  motions  of  bodies  to 
the  present  day,  not  only  in  the  solar  system  but  in  the  uni- 
verse at  large.  It  may  therefore  be  called  the  law  of  universal 
gravitation. 


2  Later   observations   did    not  confirm   the    existence   of  nebulosity,    nor   of  the 
satellite,  and  one  of  the  observations  which  produced  a  parabola  was  found  to  be 
defective.     The  object  has  turned  out  to  be  a  minor  planet  moving  in  an  elliptic 
orbit  of  greater  eccentricity  than  that  of  any  other  known  planet. 

3  Delivered  in  substance  also  at  the  University  of  California,  March  23,   1915, 
as  the   Faculty   Research  Lecture  for  the  year   1914-15. 


178  THE  ADOLFO  STAHL  LECTURES 

The  law  of  gravitation  is  probably  not  the  ultimate  state- 
ment of  force  operating  in  the  universe,  but  merely  an  aspect 
of  the  same.  The  ultimate  statement  must  include  other 
phenomena,  such  as  the  pressure  of  light  established  in  physics 
and  supposed  to  be  effective  on  the  minute  particles  of  a 
comet's  tail,  and  electro-magnetic  phenomena.  We  are  not 
concerned  with  these  phenomena  in  the  study  of  the  motions 
of  planets,  satellites  and  the  nuclei  of  comets  in  our  solar  sys- 
tem. And  yet  there  exist  certain  apparently  unexplained 
discrepancies  of  motion  of  massive  bodies  which  have  cast 
doubt  on  the  absolute  correctness  of  Newton's  law.  It  may  be 
said  with  certainty  that  in  spite  of  all  that  has  been  written  to 
the  contrary,  these  discrepancies  present  no  evidence  that 
Newton's  law  of  gravitation  requires  correction,  for  the  dis- 
crepancies referred  to  are  constantly  being  diminished  by  a 
more  rigid  and  comprehensive  mathematical  translation  of  the 
law  into  motion  and  by  more  perfect  numerical  methods.  Such 
improvements  as  have  been  made  in  recent  years  by  Newcomb 
and  Brown  in  the  interpretation  of  the  motion  of  the  Moon, 
inspire  us  with  confidence  that  ultimately  all  remaining  dis- 
crepancies will  be  conquered  on  the  basis  of  Newton's  law. 

At  any  rate  until  every  possible  mathematical  device  in 
the  application  of  Newton's  law  has  been  exhausted,  we  must 
accept  its  formulation  as  sufficient  for  the  complete  explana- 
tion of  the  motion  of  massive  bodies. 

The  process  of  translating  the  law  of  gravitation  into 
motion  has  challenged  and  is  still  challenging  the  highest 
mathematical  ingenuity.  The  translation,  as  far  as  it  has  been 
accomplished,  teaches  us  that  the  apparently  irregular  motions 
of  the  bodies  as  seen  from  the  Earth  are  controlled  by  beautiful 
and  harmonious  laws,  and  these  laws  enable  us  to  determine 
the  past  and  future  motion  of  individual  bodies  and  thus  to 
interpret  the  solar  system  as  a  whole. 

As  stated  before,  according  to  Newton's  law,  every  particle 
of  matter  attracts  every  other  particle  of  matter  with  a  force 
determined  by  their  masses  and  mutual  distances.  Let  us  take 
the  case  of  only  two  masses;  and,  in  order  to  simplify  our 
consideration  still  further,  let  us  consider  a  comet  or  a  minor 


MOTIONS  IN  THE  SOLAR  SYSTEM  179 

planet  (asteroid)  moving  about  the  Sun  as  a  primary,  or  a 
satellite  (moon)  moving  about  a  planet  as  its  primary.  The 
mass  of  the  comet,  asteroid,  or  satellite  is  negligible,  and  we 
are  concerned  only  with  the  mass  of  the  primary.  These  are 
the  simplest  cases  of  the  so-called  problem  of  two  bodies.  The 
translation  of  Newton's  law  into  motion  for  the  two-body 
problem  has  been  fully  accomplished  by  Newton  and  has 
resulted  in  the  mathematical  demonstration  of  certain  laws, 
which  had  previously  been  stated  in  a  somewhat  imperfect 
form  by  Kepler,  who  had  evaluated  them  as  the  result  of  a 
lifetime  of  guesses. 

These  results  of  the  translation  of  Newton's  law  into  motion 
tell  us  that  each  planet,  comet,  or  satellite  moving  solely  under 
the  attraction  of  its  primary,  moves  in  its  own  plane,  from 
which  it  never  can  depart;  that  this  plane  passes  through  the 
center  of  the  primary ;  that  each  describes  about  the  primary  a 
curve  known  in  mathematics  as  a  conic  section,  which  may  be 
a  circle,  or  an  ellipse,  or  a  parabola,  or  an  hyperbola ;  that 
whatever  may  be  the  conic,  the  line  (radius  vector)  joining 
body  and  primary  describes  equal  areas  in  equal  times ;  finally, 
that  the  time  (period)  of  revolution  about  the  Sun  in  the  circle 
or  ellipse  depends  solely  on  the  semi-major  axis  of  the  conic,  in 
such  a  way  that  the  square  of  the  period  expressed  in  years  is 
numerically  equal  to  the  cube  of  the  distance  expressed  in 
astronomical  units  of  length,  which  unit  is  the  distance  of  the 
Earth  from  the  Sun.  When,  therefore,  the  semi-major  axis  of 
an  ellipse  or  the  radius  of  the  circle  is  given,  the  period  in 
years  becomes  known  at  once.  If  we  divide  the  circumference 
of  360°  by  the  number  of  days  in  the  period  we  obtain  the 
average  angle  described  about  the  Sun  in  one  day,  which  is 
called  the  mean  daily  motion  \L,  usually  expressed  in  seconds 
of  arc. 

These  are  the  harmonious  and  orderly  laws  that  reveal 
themselves  from  the  translation  of  Newton's  law  into  motion 
in  the  simple  case  of  the  problem  of  two  bodies.  Mathe- 
maticians have  not  as  yet  accomplished  the  complete  transla- 
tion into  motion  and  the  discovery  of  all  of  the  corresponding 
harmonious  laws  in  the  case  of  three  or  more  bodies  mutually 
attracting  one  another. 


180  THE  ADOLFO  STAHL  LECTURES 

Interpreted  in  another  way,  the  translation  of  Newton's  law 
into  motion  in  the  case  of  two  bodies  reveals  the  fact  that  the 
path  of  a  body  is  fully  described  by  what  may  be  termed 
distinct  and  independent  earmarks,  which  are  called  elements, 
and  these  earmarks  or  elements,  six  in  number,  depend  on 
certain  initial  conditions.  If  we  assume  as  initial  conditions 
that  at  a  given  instant  a  body  is  in  a  certain  position  with 
respect  to  the  Sun  and  that  it  is  projected  in  a  given  direction 
with  a  given  speed,  then  these  initial  conditions,  namely,  posi- 
tion and  velocity,  will  fully  determine  the  numerical  values  of 
the  six  earmarks  or  elements  of  the  orbit.  A  position  is 
mathematically  expressed  by  three  numbers,  or  coordinates, 


/ 


t 


PI 


\ 


FIG.  11.     THE  APPARENT  RETROGRADE  MOTION  OF  A  MINOR  PLANET  NEAR 

OPPOSITION. 

and  the  magnitude  and  direction  of  velocity  are  expressed  by 
three  other  numbers.  These  six  initial  numbers  determine  the 
numerical  values  of  the  six  earmarks  or  elements  which  result 
from  the  translation  of  Newton's  law  into  motion.  Thus  we 
see  that  six  known  numbers  or  conditions  lead  us  to  the  six 
elements  or  earmarks,  and  when  these  initial  conditions,  what- 
ever they  may  be,  are  accurately  given,  then  the  orbit  of  the 
body  as  described  by  its  elements  becomes  very  accurately 
known,  and  we  are  enabled  to  trace  the  past  and  future  motion 
of  the  body,  and  thus  the  past  and  future  of  the  system  as  a 
whole,  provided  that  for  the  present  we  restrict  ourselves  to 
the  two-body  problem.  These  matters  will  appear  a  little 
clearer  from  the  diagrams  and  tables  which  we  shall  now 
discuss.- 


MOTIONS  IN  THE  SOLAR  SYSTEM  181 

Figure  11  shows  the  Earth  and  a  minor  planet  in  two  cor- 
responding positions  E15  P±  and  E2,  P2  in  their  respective  orbits 
with  reference  to  the  Sun  S.  Both  move  in  an  easterly  direc- 
tion approximately  in  circles.  For  each  of  these  two  bodies  the 
cube  of  the  distance  from  the  Sun  expressed  in  astronomical 
units  is  equal  to  the  square  of  its  period  of  revolution  expressed 
in  years.  The  easterly  or  direct  motion  of  the  planet  around 
the  Sun  is  therefore  less  rapid  than  that  of  the  Earth.  In  the 
first  position  planet  and  Sun  are  on  opposite  sides  of  the 
Earth  and  the  planet  is  said  to  be  in  opposition.  Px  is  seen  from 
E!  in  the  position  P/  on  the  celestial  sphere.  P2  is  seen  from 
E2  at  P2',  which  is  west  of  P/.  As  seen  from  the  Earth  a 
planet  near  opposition  moving  in  a  nearly  circular  orbit  ap- 
pears to  be  moving  westward  or  in  a  retrograde  direction. 


0 

FIG.  12.    THE  LAW  OF  EQUAL  AREAS  IN  ELLIPTIC  MOTION. 

Figure  12  shows  an  ellipse.  The  shaded  sectors  illustrate 
the  law  that  equal  sector  areas  are  described  in  equal  times. 
The  shaded  areas  are  supposed  to  be  equal.  It  takes  the  body 
just  as  long  to  move  in  its  curve  from  A  to  B  as  from  C  to  D, 
from  E  to  F,  and  from  G  to  H.  At  A  when  it  is  near  to  the 
Sun  at  perihelion  its  angular  motion  around  the  Sun  S  is 
therefore  much  more  rapid  than  when  it  is  far  away  from  the 
Sun  at  aphelion.  The  areas  of  two  successive  sectors  of  the 
conic  are  proportional  to  the  times  which  it  takes  to  describe 
them.  But  the  ratios  of  the  areas  of  triangles  contained 
between  three  successive  radii  vectores,  which  form  an 


182 


THE  ADOLFO  STAHL  LECTURES 


important  part  in  our  discussion,  are  approximately  propor- 
tional to  the  intervals  only  if  these  intervals  are  comparatively 
small.  The  distance  from  S  to  A  is  called  the  perihelion  dis- 
tance (q),  because  at  A  the  body  passes  around  the  Sun.  The 
distance  from  the  center  to  either  vertex  is  called  the  semi- 
major  axis.  The  Sun  is  at  the  focus  S  of  the  ellipse.  The 
distance  of  S  from  the  center  O  divided  by  the  semi-major 
axis  is  called  the  eccentricity,  or  the  amount  that  the  Sun  is  out 
of  center.  The  size  of  the  ellipse  depends  upon  the  major 


FIG.  13.    ELLIPSE,  PARABOLA  AND  HYPERBOLA. 

axis,  but  its  shape,  whether  circular  or  flat,  depends  upon  the 
eccentricity.  The  semi-major  axis  and  the  eccentricity  form 
two  of  the  earmarks  or  elements  of  the  orbit.  For  the  ellipse 
the  eccentricity  is  less,  for  the  hyperbola  greater  than  unity, 
for  the  circle  it  is  zero  and  for  the  parabola  exactly  unity. 

Figure  13  shows  an  ellipse,  a  parabola,  and  an  hyperbola. 
The  ellipse,  as  we  saw  before,  is  closed.    The  parabola  is  a 


MOTIONS  IN  THE  SOLAR  SYSTEM 


183 


limiting  ellipse,  which  is  closed  at  infinity.  The  period  of  a 
revolution  of  a  body  moving  in  a  parabola  is  infinite,  and 
therefore  such  a  body  after  visiting  the  Sun  will  return  only 
after  an  infinite  time,  or  not  at  all.  The  hyperbola  also  is  a 
curve  which  passes  off  to  infinity,  so  that  a  body  moving  in  it 
will  never  return.  It  is  clear  at  once  that  bodies  moving  in 
parabolas  or  hyperbolas  might  be  considered  as  visitors  from 
outer  space,  which  after  revolving  around  the  Sun  again 
disappear  into  space.  Whether  a  body  will  describe  an  ellipse 
or  a  parabola  or  a  hyperbola  depends  upon  the  initial  conditions 
referred  to  before,  namely,  on  its  position  and  velocity  at  a 
given  time. 


FIG.   14.     Two  ELLIPSES,  SHOWING  How  THE  ANGLE  o>  DEFINES  THE 
POSITION  OF  AN  ORBIT  IN  ITS  OWN  PLANE. 

Figure  14  exhibits  two  ellipses.  The  axes  lie  in  different 
directions  with  reference  to  the  horizontal  line.  The  angle 
between  a  fixed  reference  line  and  the  axis  is  another  earmark 
of  the  orbit.  It  tells  us  how  the  orbit  lies  in  its  own  plane. 
The  fourth  earmark  is  given  by  the  date  of  perihelion  passage. 
These  earmarks  are  expressed  by  symbols  in  astronomy  as 
follows:  the  semi-major  axis  by  the  letter  a;  the  eccentricity 
by  the  letter  e\  the  angle  which  the  semi-major  axis  makes 


184  THE  ADOLFO  STAHL  LECTURES 

with  the  reference  line,  usually  the  line  of  nodes,  to  be  defined 
presently,  by  the  Greek  letter  co ;  the  time  of  perihelion  passage 
by  T.  With  these  four  earmarks  and  with  the  aid  of  Kepler's 
improved  laws  it  is  possible  to  determine  the  position  of  the 
body  in  its-  orbit  at  any  given  time.  In  place  of  a  we  may 
choose  the  period  P  or  the  mean  motion  fi  as  the  first  element. 
For  a  parabola  the  perihelion  distance  q  is  chosen  as  element 
in  place  of  the  infinite  semi-major  axis.  It  remains  to  deter- 
mine the  position  of  the  body  in  space. 


FIG.  15.     THE  ORBIT  OF  THE  EARTH  AND  OF  A  COMET. 

In  Figure  15  we  see  the  Earth's  orbit.  Its  plane  may  be 
taken  as  a  reference  plane.  Then  we  see  the  plane  of  the  orbit 
of  a  comet  which  in  this  case  is  a  parabola.  The  orbit  planes 
intersect  in  a  straight  line  called  the  line  of  nodes  and  at  a 
given  angle.  The  point  where  the  comet  crosses  the  ecliptic 
from  south  to  north  is  called  the  ascending  node.  The  angu- 
lar distance  of  the  node  from  some  fixed  point  in  the 
Earth's  orbit,  generally  the  Vernal  Equinox,  V,  is  designated 
by  the  Greek  letter  Q,  while  the  angle  of  inclination  is 
designated  by  the  letter  i.  These  six  elements  or  earmarks 
are  the  numerical  characteristics  or  constants  which  distinguish 
the  orbits  of  different  bodies,  according  to  Kepler's  and 
Newton's  laws. 

The  accuracy  with  which  the  numerical  values  of  the 
elements  can  be  calculated  depends  not  only  on  the  accuracy 
of  the  given  initial  conditions,  but  also  on  a  number  of  other 
factors.  What  are  these  given  conditions,  in  practice?  They 


MOTIONS  IN  THE  SOLAR  SYSTEM  185 

were  stated  before  in  terms  of  the  position  and  velocity  of  the 
body  with  reference  to  the  Sun  at  a  given  time,  but  these 
initial  conditions  cannot  be  known  at  first  hand. 

When  a  new  object  is  discovered,  it  is  observed  from  the 
Earth,  which  itself  is  in  motion  around  the  Sun.  At  a  par- 
ticular instant  it  is  seen  projected  on  the  sky  in  a  certain 
direction.  Nothing  is  known  about  its  distance  either  from 
the  Earth  or  from  the  Sun.  Its  position  on  the  celestial  sphere 
is  defined  exactly  as  a  point  is  defined  on  the  Earth  in  geogra- 
phy. On  the  Earth  we  locate  a  point  by  its  geographical 
longitude  and  latitude.  Two  similar  arcs  or  angles  (spherical 
coordinates)  are  used  to  locate  the  point  at  which  the  body  is 
seen  on  the  celestial  sphere.  Referred  to  the  celestial  equator 
they  are  called  right  ascension  (a)  and  declination  (51 
instead  of  longitude  and  latitude.  Every  observation  of 
direction  thus  gives  us  two  angles,  an  a  and  a  5,  or  in  our 
former  language,  two  initial  conditions.  Our  translation  of 
the  law  of  gravitation  into  motion  has  shown  us  that  we  need 
six  of  these  conditions.  Therefore  the  body  must  be  observed 
at  least  at  three  different  times  to  furnish  the  necessary  initial 
conditions  for  the  determination  of  the  elements. 

The  process  of  transforming  observed  positions  (a,  5) 
into  elements  is  called  an  orbit  method. 

In  the  problem  of  two  bodies,  we  are  dealing  with  two 
distinct  aspects  of  the  study  of  motion.  The  one  is  the 
symbolic  mathematical  translation  of  Newton,  demonstrating 
Kepler's  laws  without  numerical  calculation,  which  reveals  the 
general  laws  of  motion  and  the  geometrical  nature  of  the 
elements.  The  other  is  the  derivation  of  orbit  methods  and 
the  numerical  calculation  of  the  elements  of  the  orbit.  In  the 
case  of  the  two-body  problem  the  mathematical  derivation  of 
the  general  laws  from  the  law  of  gravitation  is  comparatively 
easy,  but  the  derivation  of  the  elements  of  an  orbit  from 
observed  conditions,  such  as  right  ascensions  and  declinations, 
involves  enormous  mathematical  intricacies. 

The  foregoing  statement  brings  us  to  state  the  subject  of 
our  present  discourse.  We  shall  be  concerned  this  evening 
with  some  considerations  regarding  improvements  ac- 
complished in  the  methods  of  orbit  determination  and  we  shall 


186  THE  ADOLFO  STAHL  LECTURES 

illustrate  our  methods  by  the  results  of  recent  applications  at 
the  Students'  Observatory  of  the  University  of  California. 

The  significant  fact  about  all  of  the  methods  so  far  in 
general  use  is  that  they  are  indirect.  Since  the  distances  of 
the  observed  object  from  the  Earth  or  the  Sun  are  unknown 
at  the  outset,  an  assumption  regarding  them  is  made  in  the 
older  methods.  A  further  assumption  consists  in  considering 
the  ratios  of  the  triangular  areas  referred  to  above  propor- 
tional to  the  corresponding  intervals.  With  these  assumptions 
elements  are  determined  numerically.  As  a  check  the  positions 
are  then  computed  from  the  elements  for  the  dates  of  observa- 
tion. If  the  computed  do  not  check  with  the  observed  positions, 
new  assumptions  are  made  and  the  approximations  are  kept 
up  until  the  problem  is  satisfactorily  solved.  Therein  consists 
the  indirectness  of  the  methods. 

Following  certain  principles  originated  by  Laplace,  certain 
direct  methods  have  now  been  devised  which  are  practically 
free  from  assumptions  and  which  admit  of  the  determination 
of  the  distances  by  direct  computation,  and  thus  also  lead 
directly  to  the  desired  results.  It  is,  of  course,  impossible  to  go 
into  the  details  of  the  principles  of  these  methods,  but  we  may 
briefly  summarize  what  they  accomplish  as  follows : 

We  can  perform  a  very  simple  general  solution  without 
the  previously  customary  assumption  regarding  the  nature  of 
the  conic  or  regarding  the  nature  of  the  object,  whether  in 
doubtful  cases  it  is  a  comet  or  a  planet  or  a  satellite.  When 
any  kind  of  an  assumption  is  made  the  solution  is  called  a 
conditioned  solution.  But  if  warranted  we  may  follow  the 
traditional  method  of  assuming  the  orbit  to  be  a  parabola  or  a 
circle  or  an  ellipse  of  assumed  period,  and  we  may  then  test 
the  feasibility  of  the  assumption  in  the  course  of  the  solution. 
The  distance  of  the  body  from  the  Earth  is  found  by  direct 
computation.  In  a  conditioned  solution  a  simple  geometrical 
device  furnishes  this  geocentric  distance,  and  in  a  general 
solution  its  accurate  value  is  taken  from  a  table.  The  moment 
the  distance  is  known  the  solution  for  the  elements  becomes 
comparatively  simple.  New  mathematical  expressions  make 
the  solution  possible  in  many  cases  where  the  older  methods 
fail.  The  numerical  accuracy  of  the  results  may  be  determined 


MOTIONS  IN  THE  SOLAR  SYSTEM  187 

in  advance.  When  a  number  of  mathematical  solutions  arise 
it  is  now  possible  to  discriminate  the  orbit  in  which  the  body 
actually  moves.  The  new  formulae  admit  of  the  transition 
from  a  general  to  a  conditioned  solution  without  much  extra 
labor,  while  in  the  older  methods  a  change  of  assumption 
regarding  the  nature  of  the  conic  requires  an  entirely  new 
process  of  computation.  The  effects  of  displacement  of  the 
observed  body  as  seen  from  different  points  on  the  Earth 
(parallax)  is  readily  overcome.  Immediate  account  may  be 
taken  of  the  attractions  of  other  bodies  besides  the  Sun. 

The  earlier  orbit  methods  for  the  two-body  motion  are 
based  on  Newton's  previous  translation  of  his  law  into  motion, 
resulting  in  the  perfection  of  Kepler's  laws,  with  the  con- 
sequent definition  of  the  elements.  This  analytical  solution  was 
thereafter  applied  without  due  consideration  of  the  conditions 
of  individual  cases.  Prejudiced  by  the  fact  that  in  the  two- 
body  problem  the  analytical  solution  preceded  the  derivation  of 
orbit  methods,  astronomers  have  held  that  it  would  not  be 
possible  to  accomplish  a  direct  solution  of  the  orbit  of  a  body 
moving  simultaneously  under  the  attraction  of  more  than  one 
mass,  as  of  Jupiter  and  the  Sun,  such  as  is  the  case  with  the 
6th,  7th,  8th  and  9th  Satellites  of  Jupiter  discovered  within 
recent  years,  until  the  translation  of  Newton's  laws  into  motion 
should  have  been  accomplished  for  three  or  more  bodies  mov- 
ing under  their  mutual  attractions.  The  orbit  computations 
made  elsewhere  on  the  6th,  7th  and  8th  satellites  of  Jupiter, 
and  on  the  9th  satellite  of  Saturn,  were  based  on  a  very  large 
number  of  observations  extending  over  several  months,  and 
involved  months  of  extraordinary  labor.  Even  then  the  inves- 
tigators stated  that  inasmuch  as  no  method  existed  for  the  solu- 
tion of  cases  of  this  sort,  the  determination  of  the  orbits  was 
almost  impossible.  It  was  accomplished  finally  only  by  varying 
assumptions  regarding  one  or  more  of  the  elements  and  mak- 
ing the  others  fit  the  observations. 

Without  attempting  to  go  into  the  history  of  the  derivation 
of  the  new  methods  of  handling  these  cases,  it  may  suffice  to 
state  that  in  order  to  determine  the  orbit  of  a  body  moving 
under  the  attraction  of  a  major  planet  and  of  the  Sun,  it  is 
not  necessary  to  wait  for  the  mathematical  translation  of 


188  THE  ADOLFO  STAHL  LECTURES 

Newton's  law  into  motion  for  three  or  more  bodies  which  has 
not  as  yet  been  accomplished.  On  the  contrary  it  has  been 
possible  to  derive  a  method  whereby  accurate  results  may  be 
obtained  with  comparatively  little  trouble  from  only  three  or 
perhaps  five  observations  taken  at  limited  intervals  of  a  few 
days.  It  is  thereby  possible  to  preserve  a  discovery  on  the 
basis  of  scant  observational  material  before  the  body  may 
disappear.  These  advances  will  be  illustrated  by  means  of 
results  obtained  at  Berkeley  in  recent  years. 

Immediately  following  the  discovery  of  a  new  object,  it  is 
of  prime  importance  to  predict  its  motion  for  the  immediate 
future  from  the  first  three  available  observations,  generally 
secured  in  different  parts  of  the  world  and  transmitted  by 
telegraph.  For  this  purpose  preliminary  elements  are 
calculated  first  from  the  observed  a  and  6,  and  then  positions 
(a,  6)  are  computed  from  the  elements  at  equidistant  dates 
following  the  observation.  The  tabulated  values  of  the  posi- 
tions (a,  6)  form  an  ephemeris,  which  together  with  the 
elements  is  distributed  by  telegraph  all  over  the  world  so  that 
observers  may  locate  and  observe  the  body.  The  accuracy  of 
the  predicted  ephemeris  depends  on  the  accuracy  of  the 
elements  from  which  it  is  computed.  The  constant  aim  of 
theoretical  astronomers  is  to  devise  and  employ  orbit  methods 
which  yield  the  most  accurate  results  from  observations  made 
in  quick  succession,  even  less  than  a  day  apart,  with  the  least 
expenditure  of  time  in  calculation.  In  any  orbit  method  the 
accuracy  of  the  elements  increases  t>f  course  with  the  length 
of  time  that  the  body  has  been  under  observation  and  with  the 
accuracy  of  the  observations.  If  one  orbit  method  yields  from 
a  short  arc  elements  and  an  ephemeris  comparable  in  accuracy 
with  results  requiring  a  longer  arc  in  another  method,  then  the 
former  method  becomes  more  satisfactory  than  the  latter. 
Comparisons  of  many  orbits  computed  at  Berkeley  with  those 
derived  by  older  methods  elsewhere  lead  inevitably  to  the 
conclusion  that  the  Berkeley  results  are  more  satisfactory.  A 
typical  case  is  exhibited  in  Table  1.  In  this  table  the  differences 
AT,  Aco,  etc.,  of  the  elements  T,  co,  etc.,  of  Comet  d  1907 
(Daniel),  from  the  best  available  elements,  in  this  case  by 
Kritzinger  from  an  arc  of  74  days,  are  given,  to  show  how 


MOTIONS  IN  THE  SOLAR  SYSTEM 


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190  THE  ADOLFO  STAHL  LECTURES 

close  various  preliminary  orbits  computed  from  short  arcs, 
observations  taken  a  few  days  apart,  approach  the  best  orbit 
finally  available  for  reference. 

In  another  case,  Comet  e  1909  (Daniel),  a  European  astron- 
omer, employing  the  older  orbit  methods,  required  an  arc  of 
37  days  to  obtain  as  accurate  an  ellipse  as  that  obtained  by 
\  Messrs.  S.  Einarsson  and  R.  K.  Young  at  Berkeley  from  a 
7-day  arc,  while  other  investigators  were  still  satisfied  with  a 
parabola  from  a  similar  arc.  In  still  another  case  a  European 
astronomer,  after  computing  several  orbits  for  Comet  b  1910 
(Metcalf)  from  arcs  of  increasing  length,  finally,  by  the  use 
of  an  arc  of  37  days,  reproduced  the  Berkeley  results  obtained 
by  Mr.  Young  from  a  2-day  arc. 

A  very  interesting  case  is  that  of  Comet  e  1910  (  Cerulli  - 
Faye).  This  comet  was  discovered  by  Cerulli  in  Italy,  and 
first  observed  November  9  at  Rome.  In  addition  two  other 
observations  at  2-day  intervals  were  available.  The  usual 
direct  solution  for  a  parabola  by  Mr.  W.  F.  Meyer  and  Miss 
Sophia  H.  Levy  at  Berkeley  gave  elements  which  showed  some 
similarity  to  those  of  Comet  Faye,  which  had  been  considered 
lost.  At  the  time  of  the  discovery  of  the  Cerulli  comet  the  lost 
comet  Faye  was  an  object  of  intense  interest  to  the  astronomi- 
cal world  for  the  reason  that  astronomers  failed  to  find  it  in  its 
predicted  place  in  1903.  The  comet  was  originally  discovered 
by  Faye  in  1843,  and  was  observed  in  seven  successive  returns, 
until  1896,  its  period  of  revolution  being  about  seven  and  a  half 
years.  A  very  accurate  orbit  had  been  derived  on  the  basis  of 
the  observations  of  its  first  four  appearances  up  to  1866,  by 
taking  into  account  the  disturbing  attractions  of  the  major  plan- 
ets of  the  solar  system.  It  had  been  found  later,  at  every  return 
following  1866  up  to  1896,  by  bringing  forward  the  earlier  cal- 
culations, but  without  taking  account  of  the  disturbing  action 
of  the  major  planets.  Between  1896  and  1903,  when  it  was 
again  due,  it  passed  close  to  Jupiter  and  suffered  considerable 
change  of  motion.  Predictions  for  1903  placed  it  in  an  unfa- 
vorable position  for  observation.  In  spite  of  exhaustive  search 
on  the  part  of  many  astronomers  it  could  not  be  located. 
The  fate  of  this  comet  aroused  considerable  interest.  It 
was  thought  that  it  might  have  suffered  the  fate  of  Biela's 


MOTIONS  IN  THE  SOLAR  SYSTEM  191 

comet,  which  is  supposed  to  have  disintegrated  into  a  swarm 
of  meteors.  Its  fate  was  all  the  more  puzzling  because  it  had 
been  observed  at  eight  different  returns,  and  the  numerous 
observations  taken  at  these  appearances  ought  to  have  led  to 
a  very  accurate  determination  of  the  orbit  and  an  exact  predic- 
tion of  its  position  if  the  disturbing  actions  of  the  major  planets 
were  carefully  taken  into  account.  A  calculation  of  these  pre- 
dictions, however,  is  an  enormous  task  and  may  require  years 
of  labor. 

The  importance  of  the  question  prompted  the  .Royal 
Academy  of  Sciences  of  Denmark  in  1906  to  announce  to  the 
astronomical  world  the  competitive  problem,  "To  study  in 
detail  the  orbit  of  the  periodic  comet  Faye  on  the  strict  basis  oi 
the  observations  taken  between  1873  and  1896".  A  prize  of 
four  hundred  crowns  was  to  be  awarded  to  the  investigator 
whose  calculations  should  lead  to  the  rediscovery  of  the  comet 
which  again  was  due  in  1910.  This  was  the  situation  when  it 
was  recognized  at  Berkeley  that  the  parabolic  elements  of 
Cerulli's  comet  bore  such  a  close  resemblance  to  those  of  the 
lost  comet  that  the  identity  seemed  probable.  Since  the  original 
parabolic  solution  could  be  at  once  modified  into  a  general 
solution  without  hypothesis  as  to  the  eccentricity  of  the  orbit, 
and  since  our  methods  enable  us  to  form  a  fairly  reliable 
opinion  as  to  the  accuracy  of  a  general  solution,  this  general 
solution  was  undertaken  by  Mr.  Meyer  and  Miss  Levy,  and 
resulted  in  elements  which  were  so  nearly  identical  with  those 
of  the  lost  Faye  comet  that  announcement  could  be  made  with- 
out hesitation  that  the  new  comet  discovered  by  Cerulli  was 
identical  with  the  supposedly  lost  Comet  Faye.  The  solution 
of  the  competitive  problem  will  be  unnecessary  for  the  time 
being. 

The  adoption  of  the  periodic  orbit  in  this  case,  based  on  a 
general  solution  from  a  very  short  arc,  was  justified  by  the 
important  innovation  of  determining  the  possible  limits  of  the 
period  from  the  observations. 

In  September,  1913,  the  Russian  astronomer  Neujmin 
announced  the  discovery  of  a  starlike  object  which  had  the 
appearance  of  a  minor  planet.  Later,  after  careful  observation 
with  large  telescopes,  the  object  was  found  to  be  a  comet.  The 


192  THE  ADOLFO  STAHL  LECTURES 

starlike  appearance  of  this  comet  gives  rise  to  the  suspicion  that 
some  of  the  objects  which  have  been  announced  as  asteroids 
and  for  which  orbits  have  been  computed  on  the  elliptic  hypoth- 
esis with  very  uncertain  results,  may  after  all  be  comets  moving 
in  nearly  parabolic  orbits,  which  would  account  for  the  fact  that 
they  have  never  been  observed  again.  It  will  be  an  interesting 
problem  to  run  down  these  cases  which  have  given  rise  to 
much  theoretical  speculation,  and  thereby  to  solve  many  a 
mystery  existing  in  regard  to  the  motion  of  these  bodies. 
Comet  Neujmin  being  of  a  starlike  appearance  admitted  of  a 
high  degree  of  accuracy  in  the  observed  positions.  When, 
therefore,  on  the  basis  of  the.  usual  parabolic  hypothesis,  we 
found  so  slight  a  discrepancy  as  13"  in  the  representation  of 
one  of  the  observations,  we  felt  confident  that  this  discrepancy, 
however  slight,  could  be  accounted  for  only  by  the  fact  that 
the  object  was  moving  in  a  decidedly  elliptic  orbit.  A  tran- 
sition was  then  made  by  Messrs.  Einarsson  and  Nicholson  to  a 
general  solution,  which  resulted  in  the  first  orbit  seen  in  Table 
2.  Later,  on  the  basis  of  a  longer  arc  extending  from  Sep- 
tember 9  to  October  17,  or  38  days,  a  more  exact  orbit  could 
be  determined,  which  was  found  to  be  in  remarkable  agree- 
ment with  our  first  results  from  1-day  intervals. 

Table  2.— Comet  c  1913   (Object  Neujmin). 

Sept.  6,  7,  8.  Sept.  9,  22,  Oct.  17. 
co    348°  53.8'  co    346°  13'    16.8" 

Q    347    57.5  Q    347    53    55.5 

i      15      6.3  i      14    50    45.2 

$      50    22.4  $      50    53    41.1 

M-    203.40"  M,    199.001" 

The  announcement  of  a  periodic  orbit  from  1-day  inter- 
vals created  considerable  doubt  as  to  the  correctness  of  our  con- 
clusions. This  is  the  first  time  in  the  history  of  astronomy  that 
such  a  result  has  been  obtained  from  so  short  an  arc.  It  has 
been  the  custom,  and  it  is  still  customary  as  a  rule  in  astro- 
nomical circles,  to  adhere  to  the  hypothetical  parabola  until  the 
disagreement  between  observations  and  predictions  becomes  so 
striking  that  astronomers  are  forced  to  depart  from  the  parab- 
ola. The  preliminary  daily  motion  of  203"  differed  only  by 
4"  from  the  final  value,  199".  European  astronomers  published 
results  as  follows:  From  the  same  dates,  a  parabola  which 


MOTIONS  IN  THE  SOLAR  SYSTEM  193 

was  accepted  in  preference  to  our  ellipse;  from  September  6, 
8,  11,  or  a  5-day  arc,  another  astronomer  derived  a  mean 
motion  of  390";  from  September  6,  8,  12,  or  a  6-day  arc, 
another  astronomer  a  mean  motion  of  376" ;  and  finally  another 
astronomer,  by  extending  the  observations  from  September  6 
to  October  9,  or  33  days,  obtained  the  result  of  195",  which 
was  comparable  in  accuracy  with  that  obtained  at  Berkeley 
from  the  first  three  observations. 

Comet  a  1910,  discovered  January  16,  1910,  in  South 
Africa,  created  intense  interest  by  its  great  brilliancy,  rivaling 
that  of  Halley's  Comet.  Owing  to  its  brightness  orbits  were 
computed  by  many  astronomers,  but  the  results  differed  so 
enormously  as  to  attract  profound  attention  in  the  astronomical 
world.  At  Berkeley,  unfortunately,  the  usual  plan  of  calculat- 
ing a  preliminary  orbit  could  not  be  adhered  to,  because  it  had 
been  arranged  with  the  Lick  Observatory  that  the  attention  of 
the  astronomers  there  should  be  given  to  spectroscopic  ob- 
servations, since  it  was  thought  that  on  account  of  its  bright- 
ness the  comet  would  be  so  frequently  observed  elsewhere  that 
abundant  observations  would  be  telegraphed.  But  none  were 
received,  and  at  Berkeley  the  comet  was  hidden  from  the 
range  of  the  instruments  by  trees  surrounding  the  observatory. 
When  the  widely  discrepant  orbit  computations  became  known, 
Dr.  Curtis  secured  three  photographic  positions  for  us  at  the 
Lick  Observatory,  which  yielded  the  correct  orbit  without  diffi- 
culty, the  calculations  being  made  by  Mr.  Meyer  and  Miss  Levy. 

Professor  Tscherny  of  Warsaw,  Russian  Poland,  the  city 
about  which  much  has  been  heard  during  the  war,  recognized 
that  there  was  some  system  in  the  discrepancies  of  the  orbits  in 
that  they  could  be  arranged  into  three  groups  in  which  the 
individual  orbits  were  fairly  consistent.  He  at  once  suspected 
the  reoccurrence  of  a  phenomenon  noted  only  once  before  in 
the  history  of  astronomy,  namely,  that  the  mathematical 
solution  of  the  orbit  might  give  three  distinct  parabolas, 
different  computers  having  obtained  one  or  another  of  the  three 
possibilities.  Theoretically,  the  possibility  of  a  triple  para- 
bolic solution  had  been  previously  established  by  the  astrono- 
mer Oppolzer,  but  no  method  of  deciding  in  which  of  the  three 
orbits  the  body  actually  moved  was  available.  It  was  supposed 
that  this  question  could  be  settled  only  on  the  basis  of  con- 


194  THE  ADOLFO  STAHL  LECTURES 

tinued  observation.  At  the  time  of  discovery  of  this  comet  we 
had  in  press  a  simple  method  of  ascertaining  whether  a  single 
or  triple  parabolic  solution  was  possible  in  a  given  case  with  a 
test  whereby  without  further  observations  it  could  be  decided 
which  of  the  three  orbits  was  the  true  one.  If  this  paper  had 
been  off  the  press  at  the  time  of  the  discovery  of  Comet  a  1910. 
astronomers  might  have  been  saved  much  trouble.  Hereafter, 
in  a  case  of  triple  parabolic  solution  the  correct  physical 
solution  may  readily  be  ascertained. 

The  method  of  eliminating  the  two  purely  mathematical 
solutions  in  the  case  of  a  triple  solution  is  so  simple  that  I 
cannot  refrain  from  stating  it  briefly.  It  can  be  shown  that 
when  the  body  is  supposed  to  move  in  a  parabola  there  can  be 
either  one  or  three,  but  not  two,  parabolic  solutions  for  the 
geocentric  distance  of  the  comet.  If  one  of  the  mathematical 
parabolic  solutions  is  the  correct  physical  solution,  within  the 
limits  of  accuracy  attainable  for  the  orbit,  that  particular 
parabola  should  also  reveal  itself  by  making  a  general  solution 
without  parabolic  assumption.  It  can  be  shown  that  there  can 
be  one  or  two,  but  not  three,  general  solutions  for  the  geocen- 
tric distance.  Hence  only  one  of  the  parabolic  solutions  can 
agree  with  the  general  solution  within  the  accuracy  of  the  cal- 
culation. All  the  computer  has  to  do  then  is  to  get  the  three 
approximate  parabolic  geocentric  distances,  which  is  done  by 
an  easy  geometrical  device,  to  compare  the  same  with  the  two 
general  solutions  to  be  taken  from  a  special  table  of  geocentric 
distances,  and  to  select  that  parabolic  geocentric  distance  which 
is  consistent  with  a  geocentric  distance  from  the  general  solu- 
tion. If  none  of  the  parabolic  solutions  is  consistent  with  a 
general  solution,  then  all  three  must  be  rejected  and  a  general 
solution  must  be  adopted  as  in  the  case  of  Faye's  comet.  This 
process  is  exhibited  in  Table  3  for  Comet  a  1910  from  computa- 
tions by  Miss  Levy. 

Table    3. — Multiple    Orbit    Solutions    for   the    Geocentric    Distance    of 

Comet  a   1910. 
(In  astronomical  units.) 

Parabolic  Solutions 

(1)   1.02     (2)  0.86     (3)  0.63 

Tabular  General  Solutions. 

(1)   1.09     (2)  0.88 


MOTIONS  IN  THE  SOLAR  SYSTEM  195 

With  regard  to  the  uncertainty  of  the  solutions,  there  is 
agreement  only  between  the  second  parabolic  solution,  for 
which  the  geocentric  distance  =  0.86,  and  the  second  general 
solution,  for  which  the  geocentric  distance  =  0.88.  The  differ- 
ence 0.02  is  comparable  with  the  computed  uncertainty  of  the 
general  solution.  Both  of  the  other  parabolic  solutions  are 
therefore  to  be  discarded. 

It  is  now  possible  to  determine  the  correct  periodic  orbit 
in  a  variety  of  ways,  while  adherence  to  the  older  methods 
might  result  in  an  erroneous  parabolic  orbit.  In  Table  4  we 
have  a  case  for  which,  on  the  basis  of  an  intermediate  orbit  so 
chosen  from  a  previous  approximate  knowledge  of  the  orbit 
as  to  actually  represent  the  second  observed  position,  the  dis- 
crepancies or  residuals,  Ace,  AS,  between  the  observed  (O)  and 
computed  (C)  positions  are  distributed  over  the  first  and  third 
observed  places.  By  a  process  of  differential  correction  the 
intermediate  orbit  may  be  improved  so  as  to  produce  a  perfect 
agreement  between  observed  and  calculated  positions.  In  mak- 
ing this  improvement  we  may  make  a  conditioned  solution  on 
the  basis  of  a  parabolic  hypothesis,  or  we  may  make  a  general 
solution.  When  a  conditioned  solution  is  made  the  computa- 
tion can  be  arranged  in  such  a  way  that  if  the  parabola  is  not 
the  true  orbit  this  fact  shall  reveal  itself  by  an  exorbitant  dis- 
agreement in  one  of  the  coordinates,  as  for  instance  the  first 
declination.  In  the  table  it  is  shown  that  the  parabolic  hypothe- 
sis leaves  an  intolerable  discrepancy  of  more  than  11'  in  the 
first  declination,  so  that  this  hypothesis  must  be  rejected.  The 
solution  was  then  completed  by  Professor  Crawford  and  Mr. 
A.  J.  Champreux  without  hypothesis  and  resulted  in  an  ellipse 
with  a  period  of  6%  years,  which  satisfied  observations  exactly. 

Table   4.— Differential   Correction   of    Orbit   of    Comet  e  1906  (Kopff). 

Dates  of  Observation. 

(I)  August  24,  (II)  S-eptember  5,  (III)  September  15. 
Residuals  of  Intermediate  Orbit. 
I.  III. 

ro— o          Aa       ~  4'  23-5"     +0'  2ao" 

A8         -  5    54.1        —3   40.7 

Residuals  After  Differential  Correction  on  Parabolic  Hypothesis. 

I.  III. 

CO— C)  Aa         -  °'  39'3"      +0/    4>1" 

A8        —11   49.7       —0  30.7 

For  the  general  solution,  eccentricity  =  0.52,  period  =  6%  years. 


196  THE  ADOLFO  STAHL  LECTURES 

A  preliminary  orbit  of  Comet  b  1912,  discovered  by  Schau- 
masse,  was  calculated  by  Fayet  at  Nice  before  the  observations 
necessary  for  a  computation  had  reached  Berkeley.  Fayet  an- 
nounced a  similarity  of  the  parabolic  elements  with  those  of 
Tuttle's  comet,  which  had  appeared  13^2  years  previously.  A 
new  process,  in  the  nature  of  a  conditioned  solution,  was  then 
applied  by  us.  The  interval  between  the  perihelion  passages  of 
the  two  comets  which  are  suspected  to  be  identical  is  assumed 
to  be  a  multiple  of  the  period.  In  order  to  test  this  new  princi- 
ple, a  general  solution  without  hypothesis,  such  as  was  made 
for  the  Neujmin  comet,  was  also  applied.  A  remarkable  condi- 
tion was  found  to  exist.  The  first  three  observations  made  on 
October  21,  22  and  23  could  be  satisfied  by  any  kind  of  an  orbit 
from  a  circle  to  a  parabola.  If  the  new  principle  had  the  prac- 
tical value  that  the  theory  showed  it  to  possess,  then  a  condi- 
tioned solution  of  the  orbit  of  Schaumasse's  comet,  on  the  basis 
of  the  actual  period  of  13.7  years  of  Tuttle's  comet,  should 
bring  the  new  elements  into  close  agreement  with  those  of  Tut- 
tle's. This  was  actually  found  to  be  the  case,  while  neither  a 
parabolic  nor  a  general  solution  could  have  confirmed  the  iden- 
tity of  the  two  comets  from  so  short  an  arc.  The  introduction  of 
this  principle  was  all  the  more  important  for  this  comet  because 
an  attempt  to  reproduce  the  position  of  the  new  comet  from  the 
orbit  of  Tuttle's  comet  resulted  in  a  discrepancy  of  80°  in  the 
position.  This  discrepancy  was  due  partly  to  perturbations 
which  Tuttle's  comet  had  suffered  in  the  meantime,  and  partly 
to  the  relative  positions  of  Sun,  comet  and  Earth,  which 
aggravated  any  displacement  with  reference  to  the  Sun  when 
viewed  from  the  Earth.  The  interval  between  the  dates  of 
periheleon  passage  in  1899  and  1912  corresponded  to  an 
average  mean  motion  of  263".  Later  Fayet  calculated  the 
effect  of  pertubations  on  the  original  mean  motion  of  269.6", 
and  found  this  effect  to  change  it  to  264",  in  close  agreement 
with  the  value  we  had  obtained  by  our  principle  of  identifica- 
tion without  performing  the  computation  of  the  perturbations. 
The  computations  in  this  case  were  made  by  Miss  A.  E.  Glancy 
and  Miss  Levy. 

A  similar  case  is  that  of  Comet  d  1913  (Delavan).  The 
identity  of  this  comet  with  Comet  1852  IV  (Westphal)  was 


MOTIONS  IN  THE  SOLAR  SYSTEM  197 

suspected  by  the  discoverer.  It  was  at  once  confirmed  by  our 
calculations,  and  an  exact  period  determined.  Comet  Westphal 
was  observed  for  nearly  six  months  after  July  4,  1852,  and 
Hnatek  of  Vienna  made  a  very  careful  study  of  the  orbit  from 
all  available  observations.  He  found  that  the  period  of  revo- 
lution could  not  be  determined  with  great  accuracy,  and 
therefore  made  predictions  for  the  return  on  the  assumption 
of  different  periods.  His  latest  prediction  was  confined  to 
periods  between  61.0  and  61.3  years.  These  predictions  were 
made  to  aid  in  the  search  for  the  comet  in  1913.  The  comet, 
however,  was  not  discovered  from  the  predictions,  but  it  was 
identified  with  their  aid.  The  area  of  the  sky  in  which  the 
comet  might  be  looked  for  toward  the  end  of  September  of  the 
year  of  discovery,  according  to  the  assumed  periods,  extended 
nearly  90°  by  115°.  Even  the  difference  of  period  between 
61.1  and  61.2  years  placed  the  comet  anywhere  within  an  area 
of  15°  by  40°.  The  new  comet  was  discovered  within  that  area, 
and  therefore  was  suspected  to  be  identical  with  the  Westphal 
comet.  From  the  position  and  motion  given  by  the  first  two 
observations  and  with  the  aid  of  Hnatek's  ephemerides  Mr. 
S.  B.  Nicholson  and  Miss  Anna  Kidder  established  the  period 
to  be  61.121  years.  With  this  adopted  period  the  new  principle 
of  conditioned  solution,  on  the  basis  of  an  adopted  period,  was 
applied  as  in  the  case  of  Comet  Schaumasse-Tuttle,  with  results 
which  left  no  doubt  as  to  the  identity  of  the  Westphal  and  Dela- 
van  comets. 

Table  6  shows  the  close  agreement  of  the  orbit  of  Comet 
Delavan  computed  from  observations  made  on  September  27 
and  30  and  October  4  with  the  elements  of  Comet  Westphal. 

Table  6. — Orbits  of  Comets  Delavan  and  Westphal. 

1913,  September  27,  30,  October  4. 

Comet   Westphal.  Comet  Delavan. 

T     1852  Oct.  12.  4731  Gr.  M.  T.  1913  Nov.  26.  1067  Gr.  M.  T. 

a)    57°  02'  13"  |  56°  31'  36"] 

Q  347    01    40  [  346    47   45    I 

i    40    57   24  42    33   07    f 

c    0.919990  J  0.918644 

Adopted  period  61.121  years. 
Resulting  period  61.118  years. 


198  THE  ADOLFO  STAHL  LECTURES 

A  parabolic  orbit  was  computed  by  Miss  Kidder  and  Mrs. 
S.  B.  Nicholson  for  Comet  £  1913  (Zinner)  from  the  first  three 
observations  at  one-day  intervals,  the  parabola  coming  within 
the  range  of  possible  solutions.  This  orbit  is  seen  in  the  second 
column  of  Table  7.  The  editor  of  the  Astronomische  Nachrich- 
ten  cabled  that  from  an  orbit  computed  in  Europe  he  suspected 
the  identity  of  the  comet  with  a  comet  observed  in  1900  and 
discovered  by  Giacobini.  The  best  known  period  of  the  latter 
comet  was  6.87  years,  and  for  that  period  a  conditioned  solution 
was  undertaken,  given  in  the  last  column.  This  brings  the  ele- 
ments into  closer  agreement  with  those  of  Giacobini,  and  con- 
firms the  identification.  The  characteristic  then  of  this  princi- 
ple of  identification  is  that  if  the  suspected  identity  is  correct 
a  conditioned  solution  under  assumption  of  the  proper  period 
will  bring  the  elements  of  the  new  comet  into  closer  agreement 
with  those  of  the  comet  to  be  identified  than  would  a  parabolic 
or  even  a  general  solution. 

Table  7. — Orbits  of  Comets  Zinner  and  Giacobini. 

1913,  October  23,  24,  25. 
—Giacobini,  1900  III—  — 1913  £>  (Zinner)  — 

Period  6.87  years.  Parabola.  Assum'd  Per.  6.87  yrs. 

T    1900  Nov.    28.17  T    1913  Nov.    2.48     1913  Nov.    2.10 

(o    171°  29'  (o    171°  37.3'  171°  29.1' 

Q    196    32  Q    191    36.9  195    27.3 

«      29    52  *      33    14.6  31    01.1 

q    0.9342  q    0.99894  0.97787 

e    0.74168  e    1.0  0.72968 

Table  8  furnishes  an  illustration  of  the  enormous  amount  of 
labor  that  may  be  saved  by  the  application  of  convenient 
methods  of  solution.  Professor  Kreutz  of  the  University  of 
Kiel,  a  noted  orbit  expert,  attempted  as  usual  to  pass  a  parabola 
through  the  first  three  available  observations,  taken  on  Novem- 
ber 9,  13  and  17,  or  at  4-day  intervals,  of  a  comet  discovered 
in  1892  and  known  as  Holmes's  comet.  In  defining  the  ele- 
ments of  an  orbit,  earlier  in  the  evening  I  called  attention  to 
the  fact  that  the  older  methods  involve  a  process  of  guesses. 
The  first  four  orbits  in  the  table  were  obtained  by  Professor 
Kreutz  in  this  way  with  an  enormous  amount  of  labor.  None 
of  these  parabolas  would  represent  the  observations  from  which 
they  were  calculated,  and  thus  finally  he  was  led  to  attempt  a 


MOTIONS  IN  THE  SOLAR  SYSTEM 


199 


general  solution  which  gave  him  a  fifth  and  a  correct  orbit,  the 
period  being  about  seven  years,  which  accounted  for  the  fact 
that  a  parabola  could  not  be  made  to  represent  the  observations. 
Such  a  laborious  process  is  absolutely  unnecessary  at  the 
present  time ;  it  is  not  necessary  to  proceed  as  far  as  the  compu- 
tation of  a  single  parabola,  for  by  testing  whether  the  parabola 
lies  within  the  range  of  possible  solutions,  it  could  be  eliminated 
at  the  start.  As  this  appeared  to  be  a  test  case,  Mr.  Shane  of 
the  University  of  California  and  other  students  of  the  class  in 
theoretical  astronomy  repeated  the  solution  of  this  orbit.  The 
very  first  and  direct  solution,  without  applying  corrections  so 
as  to  get  the  best  possible  result,  yielded  an  orbit  which  agrees 
very  closely  with  the  true  orbit,  for  which  we  may  take  the  one 
by  Hind,  the  last  in  the  table,  as  it  is  based  on  a  long  arc. 
When  some  outstanding  residuals  in  Mr.  Shane's  orbit  are 
removed  the  results  will  be  as  satisfactory  as  those  of  Mr. 
Kreutz  after  five  orbit  computations. 

Table  8.— Elements  of  the  Comet  1892  III   (Holmes). 
Nov.  9,  13,  17. 


(0 

Q 

i 

1 

e 

M< 

P 
(yrs.) 

Kreutz  I 

3404 

3294 

249 

190 

100 

Kreutz  II 

339.2 

332.1 

24.9 

1.83 

1.00 

Kreutz  III  

334.8 

339.6 

24.9 

1.62 

1.00 

Kreutz   IV 

3283 

3464 

251 

141 

100 

Kreutz   V  
Shane 

13.6 
280 

331.5 
3295 

20.9 
212 

2.14 
225 

0.42 
036 

500" 
536" 

7.09 
662 

Hind    .      , 

147 

331.6 

208 

214 

041 

513" 

690 

Last  orbit  from  Nov.  9,  Dec.  7,  1892,  and  Jan.  5,  1893. 

Professor  W.  H.  Pickering  of  the  Harvard  College  Observa- 
tory has  drawn  attention  to  the  fact  that  the  parabolic  elements 
of  a  comet  observed  for  a  few  days  in  1907  and  known  as 
Comet  1907  III  bear  a  remarkable  resemblance  to  those  of  a 
comet  observed  in  1858  and  known  as  1858  III.  For  this 
latter  comet  Schulhof  has  published  elliptic  orbits,  with  periods 
ranging  from  5.8  to  7.5  years,  with  a  most  probable  period 
of  6.6  years.  Pickering  adopted  a  period  of  seven  years,  and 
substituted  this  for  the  infinite  period  of  the  parabola  of  Comet 
1907  III,  without  changing  the  other  elements  of  the  parab- 


200  THE  ADOLFO  STAHL  LECTURES 

ola.  This  has  been  the  customary  procedure.  He  then 
made  predictions  from  this  combination  of  elements,  which 
failed  to  lead  to  the  rediscovery  of  the  comet.  This  is  but 
natural,  because  it  can  be  shown  that  when  one  element,  in  this 
case  the  period,  is  changed,  all  the  other  elements  must  also 
be  recomputed,  to  represent  the  given  observations.  This  may 
be  accomplished  by  a  conditioned  solution  with  assumed  period. 
Such  a  conditioned  solution,  made  by  Miss  Young  on  the 
basis  of  a  7-year  period,  brings  the  orbits  of  the  two  comets  into 
striking  resemblance.  The  identity  of  the  comets,  suspected  by 
Pickering,  therefore  becomes  exceedingly  probable. 

Aethra  is  an  asteroid,  or  minor  planet,  discovered  in  June, 
1873,  by  Watson  at  Ann  Arbor,  and  observed  for  twenty-two 
days  at  Ann  Arbor  and  at  Marseilles,  France.  Two  orbits 
were  computed  by  Watson,  one  resulting  in  a  daily  motion  of 
980"  and  the  other  in  a  daily  motion  of  846".  This  difference 
represents  such  an  uncertainty  of  the  orbit  as  to  make  it 
practically  out  of  the  question  that  the  planet  could  ever  be 
located  again  except  by  the  introduction  of  methods  which 
would  remove  the  uncertainty  of  the  resulting  orbit.  It 
actually  failed  of  rediscovery  at  its  next  opposition.  Luther 
later  computed  several  orbits,  after  an  elaborate  investigation. 
For  many  of  the  returns  Luther  made  extensive  predictions 
and  at  these  returns  an  exhaustive  search  was  made  visually, 
and  later  also  photographically,  at  many  observatories.  A 
large  area  of  the  sky  was  covered  photographically  in  the  hope 
that  the  object  might  be  found,  but  it  has  remained  lost  to  the 
present  day.  Dr.  D.  Alter,  of  the  University  of  California,  has 
undertaken  the  computation  of  a  new  orbit  from  the  original 
observations,  by  our  own  methods,  excluding,  however,  from 
consideration  all  of  Watson's  observations,  to  avoid  sys- 
tematic corrections  for  different  observers.  Watson's  second 
orbit  was  based  on  his  own  observations,  to  the  exclusion  of 
the  Marseilles  observations,  while  Luther's  results  were  based 
on  all  of  the  observations.  Dr.  Alter's  first  results,  based  on 
entirely  different  observations,  agree  almost  exactly  with  the 
second  orbit  of  Watson.  Up  to  the  present  time  forty-two 
years  have  elapsed  since  the  loss  of  this  object.  The  difference 
in  the  mean  daily  motion,  according  to  Luther  on  the  one  hand 


MOTIONS  IN  THE  SOLAR  SYSTEM  201 

and  Watson  and  Alter  on  the  other  hand,  assuming  both  orbits 
to  be  of  equal  value,  is  60"  per  day.  In  a  year  this  would 
amount  to  365',  or  roughly  6°,  and  in  42  years  to  252°,  which 
would  displace  the  object  to  such  an  extent  as  to  make  a  search 
absolutely  unavailing.  Dr.  Alter  has  thus  shown  that  there  is 
nothing  remarkable  about  the  loss  of  this  object.  Further  in- 
vestigation on  his  part  has  brought  out  the  fact  that  the  obser- 
vations permit  of  so  large  a  range  of  solutions  that  a  definite 
orbit  determination  is  not  possible  and  that  its  rediscovery 
must  be  left  to  chance. 

The  explanation  of  the  loss  of  this  planet  as  now  accom- 
plished is  all  the  more  important  because  the  mean  daily  motion 
according  to  Luther  was  904",  or  approximately  three  times 
that  of  Jupiter.  If  the  mean  motions  of  the  small  planets  are 
tabulated  in  comparison  with  that  of  Jupiter,  it  is  found  that 
there  is  none  with  so  nearly  three  times  Jupiter's  mean  motion. 
The  fact  that,  according  to  Luther,  at  one  time  a  planet  did 
exist  under  those  circumstances  and  has  since  been  lost  has 
given  rise  to  much  astronomical  speculation  as  to  the  stability 
of  such  an  orbit ;  that  is,  as  to  whether  a  body  could  continue  to 
exist  in  our  solar  system  under  such  conditions.  Dr.  Alter's 
preliminary  results  show  that  the  important  question  raised  by 
the  loss  of  Aethra  has  no  significance,  because  it  was  based  on 
uncertain  elements.  Furthermore,  there  is  absolutely  no  reason 
why,  under  the  law  of  gravitation,  planets  should  not  exist  at 
this  and  similar  gaps.  In  recent  years  the  long  existing  gap  at 
600",  which  is  twice  the  mean  motion *of  Jupiter,  has  been  filled 
by  the  discovery  of  four  minor  planets.  This  is  a  striking  ex- 
ample of  how  inaccurate  numerical  results  in  astronomy  which 
have  been  accepted  as  standard  may  lead  to  considerable  and 
exhaustive  mathematical  investigation  of  theoretical  questions, 
which  become  irrelevant  on  the  basis  of  more  accurate  numeri- 
cal data. 

There  is  some  justification  in  expressing  considerable  doubt 
regarding  the  accuracy  of  the  accepted  orbits  of  many  comets 
and  asteroids.  Since  the  majority  of  the  comet  orbits  have  been 
computed  on  the  parabolic  hypothesis,  and  since  no  test  has 
been  made  as  to  whether  they  might  be  elliptic,  except  in  a  few 
instances,  a  revision  of  the  published  elements  by  new  com- 
putations would  probably  reveal  a  somewhat  different  picture 


202  THE  ADOLFO  STAHL  LECTURES 

of  the  character  and  distribution  of  comet  orbits,  and 
similarly  of  asteroid  orbits.  This  would  require  a  revision  of 
the  theoretical  results  deduced  on  the  basis  of  the  accepted 
orbits.  The  more  accurately  and  the  longer  a  comet  is 
observed,  the  more  accurately  will  its  orbit  become  known  by 
any  method.  In  1907  I  ventured  to  assert  that  the  supposition 
that  as  a  rule  comets  move  in  parabolas  was  erroneous.  This 
gave  rise  to  considerable  discussion.  This  conclusion, 
previously  suggested  on  the  basis  of  theoretical  considerations, 
was  contradicted  by  the  accepted  orbit  statistics,  but  is  now 
universally  accepted.  It  can  be  shown  that  even  the  available 
orbit  statistics  prove  that  comet  orbits  as  a  rule  are  elliptic, 
and  parabolic  only  within  the  range  or  uncertainty  of  solu- 
tion, and  that  therefore  many  accepted  parabolas  may  on 
revision  be  found  to  be  ellipses.  In  Table  9  the  comets  are 
classified  according  to  the  years  in  which  they  were  discovered 
and  observed.  In  Table  10  they  are  tabulated  according  to  the 
length  of  time  expressed  in  days  during  which  they  have  been 
under  observation.  Table  9  shows  that  as  instruments  became 
more  accurate  in  successive  periods  the  percentage  of  parabolas 
rapidly  diminished.  Table  10  shows  that  with  the  increased 
number  of  days  of  observation  the  percentage  of  parabolas 
diminishes  even  more  rapidly.  There  can  be  no  doubt  that 
every  comet  observed  with  sufficient  accuracy  and  for  a  suf- 
ficient length  of  time  will  be  found  to  be  moving  in  an  elliptic 
orbit.  The  average  eccentricity  of  these  elliptic  orbits  is  very 
high.  The  explanation  of  high  eccentricities  lies  in  the  nature 
of  things.  Long-period  comets  cannot  be  seen  from  the  Earth 
unless  their  orbits  are  so  highly  eccentric  that  they  come  within 
the  range  of  visibility  near  perihelion.  A  large  number  of 
comets  are  probably  moving  fairly  close  to  the  Sun,  say  within 
four  or  five  times  the  distance  of  the  Earth  from  the  Sun,  but 
they  can  not  be  discovered  unless  their  orbits  are  sufficiently 
eccentric  to  bring  them  close  to  the  Sun  at  perihelion  and 
therefore  within  the  range  of  vision.  While  this  treatment  of 
orbit  statistics  practically  eliminates  the  possibility  of  para- 
bolic orbits  there  remain  in  the  list  of  accepted  orbits  a 
number  of  hyperbolas.  Recently  Thraen,.  Fayet,  Fabry  and 
Stromgren  have  investigated  these  by  tracing  them  back  in 
order  to  ascertain  whether  at  some  previous  time  these  comets 


MOTIONS  IN  THE  SOLAR  SYSTEM  203 

came  sufficiently  close  to  a  large  planet  to  have  the  original 
elliptic  orbit  changed  into  an  hyperbola.  Their  calculations 
have  led  to  a  positive  result  in  this  direction  in  every  case.  As 
a  rule,  therefore,  we  may  safely  assume  that  comets  have  been 
forever  members  of  our  solar  system,  and  that  the  number  of 
comets  that  may  visit  us  from  outside  space  is  insignificant.  If 
such  comets  do  visit  us,  they  should  move  in  hyperbolic  orbits. 

Eccentricity  of  Comet  Orbits. 

Table  9. 
Discovery  Dates.  Parabolas. 

Before  1755 99  per  cent 

1756-1845    74  per  cent 

1846-1895    54  per  cent 

Table  10. 

Duration  of  Visibility.  Parabolas. 

1-99  days  68  per  cent 

100-239  days  55  per  cent 

240-511  days  13  per  cent 

The  latest  application  of  a  method  of  determining  the 
motion  of  a  body  moving  under  the  attraction  of  both  Jupiter 
and  the  Sun  has  been  made  by  Mr.  Nicholson  of  the  University 
of  California  for  the  Ninth  Satellite  of  Jupiter,  discovered  by 
him  at  the  Lick  Observatory  in  July,  1914.  There  was  some 
doubt  whether  the  object  was  a  new  moon  close  to  Jupiter  or 
a  minor  planet  seen  in  the  direction  of  Jupiter.  A  character- 
istic feature  of  the  method  is  that  it  permits  of  a  general  solu- 
tion without  previous  assumption  regarding  any  of  the  ele- 
ments, and  without  assumption  as  to  which  is  the  primary  in 
case  more  than  one  attracting  body  is  involved.  The  method 
gives  all  possible  mathematical  solutions  simultaneously,  and 
the  physical  solution  is  readily  established.  The  fundamental 
mathematical  expression  admits  of  28  different  mathematical 
solutions  of  the  geocentric  distance  of  a  body,  but  by  a  simple 
geometrical  process,  in  the  derivation  of  which  Mr.  B.  A.  Bern- 
stein of  the  University  has  been  of  great  assistance,  the  number 
of  solutions  to  come  under  consideration  is  readily  reduced  to 
three. 

Figure  16  shows  a  somewhat  complex  curve  and  a  straight- 
line  intersection  of  the  same  in  five  points.  These  intersections 
correspond  to  the  mathematical  solutions  of  the  geocentric 


204 


THE  ADOLFO  STAHL  LECTURES 


distance  of  the  object.  The  actual  values  of  these  distances 
may  be  read  off  from  the  horizontal  axis  at  the  top  of  the  figure. 
By  drawing  perpendiculars  from  the  five  intersections  to  the 
axis  we  thus  find  the  following  five  possible  geocentric 
distances  in  astronomical  units,  in  order  from  left  to  right : 

-1.90;  ±0.00;  +3.91 ;  +4.23;  +6.85. 

In  the  curve  the  left-hand  branch  is  due  to  the  attraction  of  the 
Sun,  the  right-hand  branch  to  that  of  Jupiter.  The  negative 
and  the  zero  distances  have  no  significance  for  our  purposes. 
Orbits  were  computed  by  Mr.  Nicholson  corresponding  to  the 
three  positive  distances.  The  first  of  these  was  found  to  be 
elliptic,  the  other  two,  corresponding  to  the  larger  distances, 


FIG.  16.    DIAGRAM  FOR  THE  GRAPHIC  DETERMINATION  OF  THE  GEOCENTRIC 
DISTANCE  OF  A  CELESTIAL  BODY. 

turned  out  to  be  hyperbolic  and  on  that  account  might  have 
been  rejected  as  improbable.  As  a  final  test  on  the  validity  of 
the  ellipse  all  three  orbits  were  compared  with  a  later  observa- 
tion, with  the  result  that  the  ellipse  was  definitely  established 
and  the  object  thereby  identified  as  a  new  satellite  of  Jupiter. 

The  whole  problem  was  solved  from  three  observations 
on  an  arc  of  nine  days.  A  prediction  was  made  and  the  body 
was  found  in  the  predicted  place  a  month  later.  The  observa- 
tions were  continued  by  Mr.  Nicholson  over  two  months,  until 
September.  The  results  of  his  computations  are  exhibited  in 
Table  11,  three  different  orbits  of  Satellite  IX  being  given. 
The  first  line  gives  the  preliminary  solution  resulting  from  the 
diagram  (Figure  16),  the  second  gives  its  improvement  by 
taking  into  account  later  observations,  and  the  last  resulted 
from  the  adjustment  of  the  orbit  on  the  basis  of  all  available 
observations  and  of  the  perturbations  due  to  the  Sun.  Accord- 


MOTIONS  IN  THE  SOLAR  SYSTEM 


205 


ing  to  this  solution  the  Ninth  Moon  of  Jupiter  has  a  period  of 
revolution  of  2.18  years.  For  comparison,  the  elements  of  the 
previously  known  Eighth  Satellite  are  also  given.  Its  period 
is  2.16  years.  This  agreement  of  periods  is  very  remarkable, 
as  is  also  the  similarity  of  some  of  the  other  elements. 

Table  11.— Orbits  of  Jupiter's  Eighth  and  Ninth  Satellites. 


P 

Arc 

0) 

Q 

» 

e 

M- 

Years 

IX     Prelim,   orbit  

9  days 

71.2° 

309.4° 

157.8° 

0.16 

1134" 

3.12 

IX     Improved    orbit  

2  months 

353.7 

310.6 

156.9 

0.12 

1678" 

2.12 

IX     Perturbations 

applied  

2  months 

359.9 

310.5 

157.0 

0.11 

1626" 

2.18 

VIII  

67.8 

240.0 

144.8 

0.35 

1646" 

2.16 

One  of  the  most  remarkable  cases  was  that  of  an  object,  an 
asteroid,  known  as  1911  MT.  This  object  was  discovered  by 
Palisa,  in  Vienna,  in  October,  1911.  As  stated  before,  such 
an  object  in  opposition  ought  to  be  moving  westward,  but  this 
object  was  moving  rapidly  eastward.  This  unprecedented 
observation  was  telegraphed  to  European  observatories,  but 
on  account  of  some  omission  did  not  reach  America.  Owing 
to  its  peculiar  motion  the  object  was  lost  so  that  only  three 
observations  in  all,  taken  in  two  nights,  were  available. 
European  astronomers  attempted  solutions  on  every  available 
hypothesis.  Extended  photographic  search  was  made  at  many 
observatories,  but  the  object  remained  lost.  The  peculiar 
motion  of  the  body  was  due  to  the  fact  that  its  orbit  is  highly 
eccentric  and  that  it  was  near  perihelion  and  very  close  to  the 
Earth.  Although  further  away  from  the  Sun  than  the  Earth, 
it  moved  relatively  faster  in  angular  motion  around  the  Sun, 
so  that  its  motion  as  seen  from  the  Earth  was  direct  and  not 
retrograde.  But  this  motion  was  complicated  by  the  fact  that 
owing  to  its  nearness  to  the  Earth  it  was  seen  in  different 
directions  from  different  observatories,  on  account  of  parallax. 
Such  complications  had  never  arisen  before,  and  no  method 
existed  for  taking  account  of  this  displacement. 

At  the  1911  meeting  of  the  Astronomical  and  Astrophysical 
Society  of  America  it  was  announced  as  a  great  astronomical 
calamity  that  this  object  had  been  lost.  I  suggested  to  a  class 
of  graduate  students  that  a  satisfactory  orbit  might  be  com- 


206  THE  ADOLFO  STAHL  LECTURES 

puted  without  much  difficulty.  Two  students,  Messrs.  E.  S. 
Haynes  and  J.  H.  Pitman,  volunteered  to  undertake  the  solu- 
tion under  my  direction,  and  Mr.  Haynes  later  made  this  prob- 
lem one  of  special  investigation.  He  obtained  an  orbit  from 
the  very  meager  material,  which  was  published  in  April,  1912, 
with  the  request  that  on  the  basis  of  the  new  predictions  astron- 
omers search  their  photographic  plates.  Promptly  after  our 
published  results  reached  England  a  cable  came  announcing 
that  the  object  had  been  found  in  the  predicted  place  on  three 
plates  taken  at  the  Greenwich  Observatory. 

Many  other  investigations  of  a  similar  kind  are  constantly 
under  way  in  California  and  elsewhere.  Among  the  Berkeley 
investigations  might  be  mentioned  Professor  Crawford's  and 
Mr.  Meyer's  work  on  the  Eighth  Satellite  of  Jupiter;  Dr. 
Einarsson's  work  in  determining  orbits  of  the  so-called  Trojan 
Group  of  asteroids,  which  have  a  mean  motion  very  nearly  the 
same  as  that  of  Jupiter;  and  the  investigation  of  the  perturba- 
tions of  the  Watson  asteroids  by  the  speaker  under  the  auspices 
of  the  National  Academy  of  Sciences.  It  is  natural  for  an  in- 
vestigator to  select  those  cases  for  investigation  which  have 
given  considerable  difficulty,  or  which  hitherto  have  been  found 
impossible  of  solution. 

You  will  have  noted  that  many  persons  have  been  concerned 
in  the  theoretical  work  conducted  at  Berkeley.  The  University 
has  been  fortunate  in  the  past,  and  I  trust  will  be  equally 
fortunate  in  the  future,  in  counting  among  the  younger 
members  of  the  astronomical  staff  and  among  the  student  body 
many  capable  and  enthusiastic  workers,  women  as  well  as  men, 
without  whose  assistance  little  could  be  accomplished  in  our 
orbit  work.  Although  the  most  expeditious  and  accurate 
methods  of  solution  are  employed,  problems  of  the  kind  we 
have  discussed  require  intense  and  constant  application  on 
account  of  the  precision  demanded  and  of  the  intricacies  of 
the  calculations.  Yet  there  is  a  certain  excitement  and  thrill 
involved  in  the  expectation  of  a  satisfactory  result  as  the  cal- 
culations approach  their  conclusion,  so  that  not  infrequently 
the  computations  are  continued  throughout  the  night  when  the 
end  of  the  work  is  anticipated  for  the  following  day. 


MOTIONS  IN  THE  SOLAR  SYSTEM  207 

At  some  institutions  astronomers  are  in  position  to  devote 
all  their  time  to  investigation.  In  universities  as  a  rule  only 
such  time  can  be  given  to  investigation  as  can  be  spared  by  the 
staff  of  the  department  from  their  duties  of  instruction  and  by 
the  students  from  their  regular  studies.  If  we  were  so 
fortunate  at  Berkeley  as  to  command  the  help  of  one  or  two 
regular  research  assistants  many  additional  problems  of  im- 
portance could  be  attacked.  Perhaps  the  most  fortunate  cir- 
cumstance for  us  at  Berkeley  is  our  close  relationship  to  the 
Lick  .Astronomical  Department  of  the  University,  through 
which  we  are  able  to  receive  without  the  slightest  delay  obser- 
vations of  the  highest  precision  to  serve  as  the  basis  for  the 
solution  of  urgent  problems. 

I  have  made  no  attempt  to  emphasize  the  intellectual  and 
material  service  rendered  to  the  civilized  world  by  theoretical 
and  other  astronomical  researches,  but  it  may  be  of  interest  to 
know  that  in  the  present  war  astronomers  and  their  knowledge 
have  been  found  to  be  of  the  greatest  service.  The  Berkeley 
Astronomical  Department  takes  pride  in  the  fact  that  since 
our  entrance  into  the  war  every  member  of  our  staff  to  the 
number  of  ten  has  gone  into  service  either  directly  under  the 
colors  or  in  a  civilian  capacity  and  is  meeting  every  expectation 
in  our  country's  fight  for  liberty. 


THE  BRIGHTNESS  OF  THE  STARS,  THEIR  DIS- 
TRIBUTION, COLORS,  AND  MOTIONS1 

By  FREDERICK  H.  SEARES 

On  a  clear  night  an  amazing  spectacle  is  to  be  seen  from  the 
summit  of  Mount  Wilson — not  the  panorama  of  valley  and 
mountain  nor  the  stars  of  heaven  spread  across  the  sky,  but  the 
seemingly  innumerable  lights  which  stud  the  floor  of  the  valley 
— evidence  more  convincing,  even,  than  that  of  daylight  hours 
that  the  habitations  of  several  hundred  thousand  human  beings 
lie  below.  In  the  foreground  is  the  city  of  Pasadena,  and 
beyond,  Los  Angeles,  with  the  intervening  space  almost  con- 
tinuously illuminated;  and  still  more  remote,  the  lights  of 
adjoining  towns  and  villages  reach  out  in  slender  lines  that 
here  and  there  touch  larger  groups  along  the  coast.  From 
mountain-foot  to  coast-line,  a  stretch  of  more  than  thirty 
miles,  there  is  scarcely  a  break  in  the  continuity  of  these  con- 
spicuous evidences  of  human  life  and  activity.  In  other 
directions  are  many  isolated  groups,  some  including  only  a  few 
tiny  glittering  points  of  light,  others  larger,  though  more 
compact,  but  likewise  isolated,  except  as  the  brilliant  headlight 
of  an  electric  train  or  the  lights  of  motor  cars,  slowly  thread- 
ing their  way  across  the  valley,  suggest  and  symbolize  all  the 
intimate  relationships  that  knit  together  the  life  of  modern 
towns  and  cities. 

There  is  much  in  the  spectacle  to  touch  the  imagination, 
but  it  is  not  the  imaginative  suggestiveness  of  the  scene  that 
requires  our  interest  now,  so  much  as  a  certain  parallelism 
with  the  heavens  above.  Many  things  have  been  learned  about 
the  stars,  but  to  understand  them,  to  comprehend  and  make 
them  a  really  vital  part  of  our  knowledge  of  the  world  about 
us,  they  must  be  pictured  in  terms  of  every-day  experience, 
translated  into  language  so  familiar  that  we  give  no  thought 
to  the  medium  in  which  the  facts  are  set  before  us.  On  a 
black  and  moonless  night,  the  glittering  lights  of  the  valley  are 

1  Delivered   March   IS,    1918. 


PLATE  XLVII.    LIGHTS  IN  THE  VALLEY  BELOW  MOUNT  WILSON. 


THE  BRIGHTNESS  OF  THE  STARS  209 

not  unlike  a  glorious  constellation ;  and  the  analogies  that  may 
be  traced  between  them  and  the  distant  stars  smooth  away 
many  difficulties  which  otherwise  would  be  encountered. 

1.  THE  FIXED  STARS — EARLY  CONCEPTIONS 

To  the  ancients  the  stars  were  the  "fixed  stars,"  dis- 
tinguished from  the  wandering  stars  or  planets  by  the  fact 
that  they  hold  unchanged  their  positions  with  respect  to  one 
another.  The  objects  which  for  the  Chaldean  shepherd  com- 
prised the  constellation  of  Orion  still  appear  above  the  southern 
horizon  during  winter  evenings,  with  the  configuration  they 
had  three  thousand  years  ago.  They  were  just  glittering  points 
of  light  securely  attached  to  the  surface  of  the  celestial  vault, 
whose  daily  rotation  carried  them  about  the  stationary  Earth 
from  which  he  watched  them  rise  and  set  and  sweep  across  the 
sky. 

And  thus  the  stars  remained,  fixed,  until  two  hundred  years 
ago,  when  Halley,  in  1718,  showed  that  Sirius,  Procyon,  and 
Arcturus  had  perceptibly  changed  their  positions  with  respect 
to  neighboring  stars.  Previously  there  had  been  no  evidence 
that  the  stars  might  be  in  motion,  that  the  permanence  of  form 
so  long  attributed  to  the  stellar  firmament  would  one  day  lose 
its  meaning. 

As  to  the  size  and  distance  of  the  stars  the  ancient  mind 
could  only  speculate.  Copernicus  in  the  16th  century  said  they 
must  be  very  distant  because  they  did  not  reflect  the  motion 
of  the  Earth  in  its  path  about  the  Sun.  The  phenomenon  to 
which  he  referred  is  similar  to  what  occurs  when  one  moves 
over  the  mountain  top.  The  lights  in  the  valley  seem  also  to 
shift  about,  and  if  you  watch  attentively  you  will  find  they 
mimic  every  movement  that  you  make,  but  with  motions 
opposite  in  direction  to  your  own.  Walk  a  hundred  paces 
toward  the  west  and  the  lights  in  the  foreground  just  below 
you  shift  perceptibly  toward  the  east  with  respect  to  those 
farther  off;  return  to  the  point  from  which  you  started  and 
they  promptly  reverse  their  motions  and  retreat  to  their  former 
places.  And  you  will  note  that  the  shift  of  any  given  light 
depends  upon  its  distance.  For  those  nearest  to  you  the  motion 
is  unmistakable.  For  more  distant  lights,  though  perceptible, 
it  is  much  less  conspicuous;  but  beyond  a  certain  point  the 


210  THE  ADOLFO  STAHL  LECTURES 

unaided  eye  no  longer  sees  the  shift.  The  displacement  is  too 
minute  to  be  detected  without  instrumental  aid. 

And  just  so,  the  stars  should  mimic  the  larger  excursions 
of  the  observer  in  his  annual  motion  about  the  Sun.  But  no 
such  change  of  place  had  been  detected,  because,  as  Copernicus 
said,  the  stars  are  so  very  distant.  His  opponents,  however, 
said  this  was  only  to  be  expected,  for  since  the  Earth  did  not 
revolve  about  the  Sun,  such  a  shift  could  not  occur. 

Nevertheless,  the  Copernican  point  of  view  slowly  gained 
adherents,  and  conviction  gradually  grew  in  the  minds  of 
men  that  a  central  Sun  surrounded  by  revolving  planets  is  the 
correct  conception  of  our  solar  system.  And,  finally,  the 
precise  and  skillful  measures  by  Bradley,  which  led  to  the 
discovery  of  the  aberration  of  light,  put  the  matter  beyond  a 
doubt.  The  chances  were  shown  to  be  overwhelmingly  in 
favor  of  a  motion  of  the  Earth  about  the  Sun;  and  yet  there 
was  no  evidence  of  any  corresponding  change  in  the  postions 
of  the  stars. 

2.  THE  STARS  ARE  SUNS — EXTENT  OF  THE  STELLAR  SYSTEM 

The  accuracy  of  Bradley's  measures  was  such  as  to  reveal 
any  displacement  as  great  as  2",  and  made  it  probable,  for  a 
certain  star  at  least,  that  the  actual  shift  did  not  exceed  0.5". 
The  implications  of  this  result  are  not  at  once  apparent.  A 
second  of  arc  is  an  exceedingly  small  angle.  To  subtend  such 
an  angle  an  object,  say  a  short  rod,  must  be  looked  at  from  a 
distance  of  more  than  200,000  times  its  length.  The  difference 
in  direction  between  the  two  ends  of  a  foot  ruler  seen  from  a 
distance  of  forty  miles  is  almost  exactly  a  second  of  arc ;  and 
the  diameter  of  a  small  coin,  a  ten-cent  piece,  at  a  distance  of 
two  and  a  fraction  miles  gives  the  same  result. 

Bradley's  measures  thus  meant  that  the  distance  of  even 
the  nearer  stars  must  be  several  hundred  thousand  times  that 
between  the  Earth  and  Sun.  Although  some  conception  of 
the  magnitude  of  the  universe  had  gradually  been  developing, 
his  observations  set  a  lower  limit  to  its  size,  and  showed  that, 
at  most,  the  Sun  and  all  the  planets  could  be  but  an  astonish- 
ingly insignificant  part  of  the  stellar  system. 

Further,  objects  at  so  great  a  distance  as  the  stars  must 
possess  luminosity  of  extraordinary  intensity.  Their  intrinsic 


THE  BRIGHTNESS  OF  THE  STARS  211 

brightness  must  be  enormously  great,  otherwise  they  could  not 
be  seen.  The  apparent  brightness  of  any  source  of  light,  a 
distant  star,  or  a  light  in  the  valley  viewed  from  the  mountain 
top,  depends  upon  two  things — intrinsic  brightness  and 
distance  from  the  observer.  Intrinsic  brightness,  in  the  case  of 
ordinary  lamps,  is  expressed  in  candle  power.  With  increasing 
distance  the  apparent  brightness  rapidly  diminishes,  and  from 
the  mountain  one  is  able  to  see  in  the  remoter  parts  of  the 
valley  only  those  lights  which  intrinsically  are  most  luminous. 
Conversely,  with  some  notion  of  the  distance  of  any  light,  it 
would  be  possible,  roughly  at  least,  to  estimate  its  candle- 
power.  Thus  from  Bradley's  figures  it  was  a  matter  of  easy 
arithmetic  to  calculate  that,  in  the  average,  the  stars  must  be 
of  the  same  general  order  of  intrinsic  luminosity,  and  pre- 
sumably also  of  the  same  order  of  size,  as  our  own  Sun. 
Actually,  the  differences  from  star  to  star  are  very  great; 
nevertheless  it  was  clear  that  all  might  properly  be  regarded  as 
suns,  or  from  another  standpoint,  that  our  own  Sun  could 
justly  be  ranked  among  the  stars. 

These  conclusions  have  not  yet  reached  their  full  develop- 
ment. Evidence  of  stellar  motion  was  not  available  until  the 
beginning  of  the  18th  century,  and  it  was  more  than  a  century 
after  Bradley's  observations,  and  only  eighty  years  ago,  that 
definite  measures  of  a  star's  distance  were  first  obtained.2 
The  determination  of  stellar  motions  and  distances,  which  thus 
began  almost  within  our  own  generation,  requires  the  utmost 
skill  in  measurement,  and  became  possible  only  with  the 
development  of  precise  and  sensitive  instruments.  In  the 
meantime,  much  attention  had  been  given  to  the  brightness  of 
the  stars  as  the  only  field  of  investigation  that  could  throw 
light  upon  the  great  problem  of  the  structure  of  the  stellar 
system.  If  we  were  to  observe  and  count  the  lights  visible 
from  the  mountain,  we  could  at  least  learn  their  number  and 
their  range  in  brightness.  Combining  these  results  with  the 
directions  in  which  the  lights  are  seen,  we  could  detect 
tendencies  toward  symmetry  of  arrangement,  and  easily  dis- 
tinguish the  chaotic  straggling  village  from  one  built  in 
accordance  with  an  orderly  plan. 


2  For  an  account  of  stellar  parallaxes  and  their  measurement,  see  van  Maanen's 
article,  Publ.  Astron.  Soc.  Pac.,  30,  29,   1918. 


212  THE  ADOLFO  STAHL  LECTURES 

3.    THE  MAGNITUDE  SCALE 

Ptolemy,  in  his  catalogue  which  forms  a  part  of  the  Alma- 
gest, divided  the  stars  visible  to  the  unaided  eye  into  six  classes 
or  magnitudes,  according  to  their  brightness.  The  system  'hus 
introduced  two  thousand  years  ago  was  extended  by  later  ob- 
servers ;  in  the  19th  century  it  was  given  precise  definition,  and 
is  the  one  we  use  today.  The  magnitude  is  a  unit  used  to  ex- 
press brightness,  as  the  inch  or  yard  is  used  for  the  expression 
of  length  and  distance ;  but  note  that  it  measures  a  physiologi- 
cal perception,  namely,  the  sensation  of  brightness  produced  in 
the  eye  of  the  observer  by  the  star's  luminosity,  and  is  not  to 
be  confused  with  the  intensity  or  energy  of  the  light  causing 
the  sensation. 

Magnitudes  increase  numerically  as  the  stars  become 
fainter.  The  relation  of  magnitude  and  intensity,  of  sensation 
and  stimulus,  is  not  one  of  simple  proportionality,  but  log- 
arithmic in  character.  To  produce  the  sensations  measured  by 
the  magnitudes  1,  2,  3,  4,  etc.,  the  intensities  must  decrease  in 

a  geometrical  progression,  whose  factor  is  1/2.512. ,  and  are 

thus  proportional  to  1,  1/2.512,  1/(2.512)2,  1/(2.512)3,  etc. 
Simplifying  the  sixth  term  of  this  sequence  we  find  its  value  to 
be  exactly  1/100.  Hence,  two  stars  whose  light-intensities  are 
to  each  other  as  one  to  a  hundred  differ  in  brightness  by  five 
magnitudes.  This  simple  relation  in  round  numbers  is  the 
definition  introduced  a  generation  ago  which  placed  stellar 
photometry  on  a  precise  numerical  basis.  The  factor  1/2.512, 
the  intensity-ratio  corresponding  to  a  difference  of  one  magni- 
tude, is  a  consequence  of  the  definition.  Its  unwieldiness  is  of 
no  disadvantage,  for  the  simple  reason  that  in  practice  it  is  not 
directly  used. 

We  are  to  remember,  therefore,  that  magnitude  is  primarily 
a  measure  of  sensation,  while  light-intensity  expresses  the 
stimulus  producing  the  sensation;  and  that  a  ratio  of  about  1 
to  2.5  in  the  intensity  corresponds  to  a  difference  of  one  unit 
of  the  magnitude  scale. 

In  undertaking  measurements  of  any  kind  we  must  be 
provided  with  a  scale,  something  like  the  yardstick  with  which 
we  measure  lengths,  or  the  standard  weights  used  in  the 
ordinary  operation  of  weighing.  The  unit  of  brightness  has 


THE  BRIGHTNESS  OF  THE  STARS  213 

been  defined;  but  practically  we  require  something  more 
tangible  than  a  general  statement  of  a  dozen  words  or  more. 
For  the  actual  measurement  of  stellar  brightness  we  have 
selected  certain  stars  near  the  North  Pole,  and  by  methods 
that  need  not  be  described  here  have  determined  the  magnitude 
of  each.  These  stars  are  analogous  to  the  standard  weights 
with  which  other  objects  may  be  compared;  and,  similarly,  we 
find  the  brightness  of  any  star  by  comparing  it  with  the  stand- 
ards of  known  magnitude  at  the  Pole.  The  brightness  of  many 
thousand  stars  has  been  thus  determined.  Although  standard 
magnitudes  in  other  parts  of  the  sky  are  often  used,  the 
principle  remains  the  same. 

4.  THE  NUMBER  OF  THE  STARS 

What  conclusions  may  be  drawn  from  measurements  of 
stellar  brightness  ?  What  light  do  they  shed  upon  the  problems 
of  the  stars  ?  In  Ptolemy's  catalogue  the  brightest  stars  were 
assigned  the  magnitude  1,  but  in  the  modern  readjustment  of 
the  scale  they  are  more  nearly  of  zero  magnitude.  Roughly 
we  may  take  the  6th  magnitude  as  the  limit  of  unaided  vision, 
while  with  great  telescopes  such  as  the  60-inch  reflector  on 
Mount  Wilson  a  photographic  exposure  of  three  or  four  hours 
will  reach  the  20th  magnitude. 

The  range  of  20  magnitudes  thus  within  our  reach  is  not  an 
adequate  expression  of  what  we  really  have  to  deal  with.  The 
extremes  of  intensity,  or  of  energy,  are  vastly  more  impressive. 
Since  an  interval  of  5  magnitudes  corresponds  to  an  intensity 
ratio  of  1/100,  10  magnitudes  implies  a  ratio  of  (1/100)  X 
(1/100)  or  1/10,000;  and  it  follows,  similarly,  that  stars  of 
zero  magnitude  have  an  intensity  100,000,000  times  greater 
than  those  20  magnitudes  fainter.  The  diversity  in  the  light 
of  the  stars  is  therefore  very  great.  The  extent  to  which 
differences  in  distance  contribute  to  this  result  will  be  discussed 
later. 

Even  the  casual  observer  recognizes  that  the  faint  stars  are 
much  more  numerous  than  the  brighter  objects.  For  the  tele- 
scopic stars  our  counts  are  not  complete,  but  with  allowance 
for  this  defect,  the  totals,  for  the  whole  sky,  of  objects  brighter 


214 


THE  ADOLFO  STAHL  LECTURES 


than  each  successive  magnitude  are  as  shown  in  Table  I.3 
These  results  are  subject  to  revision,  the  numbers  in 
parentheses  being  very  uncertain,  although  their  general  order 
of  magnitude  is  probably  correct. 

TABLE  I 

TOTAL  NUMBER  OF  STARS  BRIGHTER  THAN  EACH  UNIT  OF  THE  HARVARD 
SCALE  OF  VISUAL  MAGNITUDES 


Magni- 
tude 

No.   of    Stars 

Ratio 

Magni- 
tude 

No.   of   Stars 

Ratio 

0 

3 

10 

380,200 

3.7 

v 

2.7 

1 

11 

11 

1,026,000 

3.5 

2.5 

2 

39 

12 

2,588,000 

3.4 

2.3 

3 

133 

13 

5,894,000 

3.4 

2.2 

4 

446 

14 

13,120,000 

3.3 

2.1 

5 

1,466 

15 

27,540,000 

3.2 

2.1 

6 

4,732 

16 

57,150,000 

3.2 

(1.9) 

7 

,         15,000 

, 

17 

(107,200,000) 

3.1 

(1.8) 

8 

46,240 

18 

(197,200,000) 

3.0 

(1.7) 

9 

139,300 

19 

(335,000,000) 

2.7 

(1.6) 

10 

380,200 

20 

(530,900,000) 

The  numbers  of  faint  stars  seem  astonishingly  large.  Why 
should  they  be  so  greatly  in  excess  of  the  -brighter  stars  ?  At 
first  we  knew  nothing  of  differences  in  size  and  luminosity  of 
individual  stars  nor  anything  of  their  distribution  in  space.  It 
was  natural,  therefore,  to  make  tentative  inquiries  based  on 
the  assumption  that,  intrinsically,  all  stars  are  equally  luminous 
and  uniformly  distributed  throughout  an  endless  space.  The 
fainter  stars  were  accordingly  fainter  because  they  were  farther 
away.  Initially,  this  hypothesis  was  as  plausible  as  any  other ; 

3  Derived  from  Groningen  Publication,  No.  27,  p.  63.  Beyond  the  16th  mag- 
nitude the  results  are  extrapolated,  but  receive  general  confirmation  through  un- 
completed investigations  at  Mount  Wilson. 


THE  BRIGHTNESS  OF  THE  STARS  215 

and  it  is  interesting  to  see  what  it  yields  for  the  total  numbers 
of  stars  down  to  the  limits  fixed  by  each  unit  of  magnitude. 

It  is  not  difficult  to  show  that  the  total  to  any  magnitude 
will  be  (2.512)%  or  3.98  times  the  total  to  the  next  brighter 
magnitude.  In  fact,  this  result  holds  even  when  the  individual 
stars  are  not  all  intrinsically  of  the  same  luminosity,  provided 
the  mixture  of  objects  of  different  brightness  is  the  same  at  all 
distances.  Anything  like  a  uniform  distribution  of  stars 
throughout  space,  therefore,  necessarily  implies  the  existence 
of  enormous  numbers  of  faint  stars.  It  is  like  the  old  problem 
of  shoeing  the  horse,  in  which  the  cost  of  each  succeeding  nail 
is  doubled.  The  total  is  an  incredible  sum;  but  with  the  stars 
the  numbers  increase  much  more  rapidly,  for  with  each  suc- 
ceeding magnitude  the  totals  are  quadrupled  instead  of  being 
only  doubled. 

Examining  the  ratios  of  adjacent  totals  found  by  actual 
counts,  which  are  also  given  in  Table  I,  we  find  that  in  no  case 
do  they  reach  the  theoretical  maximum  of  3.98.  For  the 
brighter  stars  they  fall  little  short  of  the  maximum,  but  near 
the  lower  limit  of  brightness  now  accessible  to  observation  the 
ratio  is  only  half  that  calculated  on  the  hypothesis  of  uniform 
distribution. 

5.    LIMITATION  OF  THE  STELLAR  SYSTEM 

What  may  we  conclude  from  this  result?  Obviously  that 
the  stars  are  not  uniformly  scattered  throughout  space,  that 
with  increasing  distance  the  number  in  a  given  volume  becomes 
less  and  less. 

In  the  vicinity  of  the  Sun  the  stars  are  most  numerous,  but 
in  the  remoter  regions  they  thin  out  gradually.  From  the 
progression  of  the  totals  given  in  Table  I  it  is  evident  that 
there  are  vast  numbers  of  still  fainter  stars,  invisible  even  with 
our  most  powerful  telescopes;  but  beyond  some  limit  of 
distance  there  seem  to  be  no  stars  belonging  to  the  aggregation 
to  which  our  counts  refer.  If  the  decrease  in  the  ratio  for 
successive  totals  continues  undiminished  beyond  the  16th 
magnitude,  there  can  be  few  if  any  stars  fainter  than  the  28th 
or  30th  magnitude;4  but  the  assumption  involved  is  very  pre- 


4  This  is  not  intended  to  apply  to  star  clusters,  spiral  nebulae,  or  possible 
aggregations  of  stars  similar  to  our  own  galactic  system  but  disconnected  from  it 
and  very  distant. 


216  THE  ADOLFO  STAHL  LECTURES 

carious  indeed.  Probably  there  is  no  definite  lower  limit ;  but 
perhaps  we  may  safely  say  that  the  total  of  all  stars  fainter  than 
the  30th  magnitude  is  relatively  very  small. 

From  simple  counts  of  the  stars  for  each  interval  of  magni- 
tude we  learn  that  our  stellar  system  is  limited,  arid  that  the 
stars  gradually  thin  out  as  we  approach  its  boundaries.  We 
have  assumed  that  the  mixture  of  stars  of  different  intrinsic 
brilliancy  is  everywhere  the  same,  and  we  have  neglected 
nothing  but  a  possible  loss  or  scattering  of  light  in  its  passage 
through  space.  The  distant  lights  of  the  valley  are  obliterated 
when  the  air  is  filled  with  mist  and  haze,  while  those  still 
visible  are  much  decreased  in  brightness;  through  the  loss  of 
light  in  the  dust-laden  atmosphere  the  content  of  the  field  of 
vision  shrinks  to  a  fraction  of  its  normal  size  and  brilliance. 
Were  light  absorbed  or  scattered  in  interstellar  space,  an  infinite 
universe  might  appear  as  our  own  really  does;  but  from 
independent  evidence  it  seems  practically  certain  that  such 
absorption  as  may  exist  is  insufficient  or  not  properly  distributed 
to  affect  appreciably  our  conclusions. 

6.  FORM  OF  THE  STELLAR  SYSTEM — RELIABILITY  OF  RESULTS 

Other  results  may  also  be  derived  from  counts  of  stars. 
Since  the  time  of  the  elder  Herschel  it  has  been  recognized 
that  the  Milky  Way  is  the  backbone  of  our  stellar  system.  Stars 
of  all  degrees  of  brightness  are  more  numerous  in  its  vicinity, 
and  it  is  clear  that  the  galactic  plane  must  be  of  great 
importance  in  all  stellar  problems. 

We  arrange  our  counts  in  zones  parallel  to  this  plane  by 
supposing  circles  to  be  drawn  about  the  celestial  sphere  parallel 
to  the  Milky  Way,  at  intervals,  say,  of  10°.  Counting  the  stars 
of  each  interval  of  magnitude  between  adjacent  circles,  we  find 
that  zones  equidistant  from  the  Galaxy,  north  and  south,  contain 
approximately  the  same  number  of  stars.  The  plane  through 
the  Milky  Way  is  therefore  a  plane  of  symmetry,  and  to 
simplify  results  we  average  the  totals  for  such  pairs  of  equi- 
distant zones.  We  thus  obtain  Table  II. 

The  first  column,  with  each  successive  column,  gives  for 
the  different  zones  results  analogous  to  those  for  the  whole  sky 
in  Table  I.  The  numbers  of  stars,  however,  now  refer  to  a 
unit  area  of  one  square  degree.  The  distance  of  the  middle  of 


THE  BRIGHTNESS  OF  THE  STARS 


217 


each  zone  from  the  galactic  plane — its  galactic  latitude — is  at 
the  head  of  the  column. 

TABLE  II 

TOTAL  NUMBERS  OF  STARS  PER  SQUARE  DEGREE  BRIGHTER  THAN  A  GIVEN 
MAGNITUDE  AT  DIFFERENT  DISTANCES  FROM  THE  MILKY  WAY 

F  GALACTIC  LATITUDE 


MAG. 

5° 

15° 

25° 

35° 

45°  i  55°  j  65° 

80° 

8.5 

3.3 

2.4 

1.9 

1.5 

1.4    1.3  !   1.3 

1.2 

9.5 

10.4 

7.3 

5.5 

4.4 

3.9    3.7  ;   3.5 

3.4 

10.5 

29 

20 

14 

11 

10     98 

8 

11.5 

81 

53 

36 

28 

23    21    19 

17 

12.5 

209 

130 

86 

63 

51    44    39 

35 

13.5 

507 

301 

192 

135 

105    88    75 

64 

14.5 

1138 

676 

398 

267 

200   160  :  132 

112 

15.5 

2483 

1479 

800 

514 

369   282   229 

195 

t 

16.5  {  5495 

3162   1585 

933 

661   501   398 

331 

17.5 

12020 

6607  ;  3090 

1660 

1148   871   692 

550 

What  first  strikes  attention  is  the  crowding  of  stars  near 
the  Milky  Way.  For  all  magnitudes  the  numbers  increase  with 
decreasing  distance  from  the  galactic  plane,  but  for  the  fainter 
stars  the  crowding  is  most  pronounced.  This  is  clearly  shown 
by  comparing  the  ratios  of  the  numbers  in  the  5°  zone  with 
those  at  80°.  Such  a  ratio  is  called  the  galactic  concentration 
for  the  magnitude  to  which  it  refers.  The  increase  in  the  con- 
centration, as  fainter  and  fainter  stars  are  included  in  the  totals, 
is  strikingly  shown  in  Table  III. 

TABLE  III 
GALACTIC  CONCENTRATION  FOR  DIFFERENT  LIMITING  MAGNITUDES 


GALACTIC 

GALACTIC 

\I  AC 

CONCENTRATION 

MAG. 

CONCENTRATION 

2.5 

2.4 

10.5 

3.7 

3.5 

2.2 

11.5 

4.7 

4.5 

2.2 

12.5 

6.1 

5.5 

2.1 

13.5 

8.0 

6.5                          2.2 

14.5 

10.1 

7.5 

2.3                          15.5 

12.7 

8.5 

2.6 

16.5 

16.6 

9.5 

3.1 

17.5 

21.9 

i 

218 


THE  ADOLFO  STAHL  LECTURES 


Here  again  the  numbers  are  subject  to  some  revision,  par- 
ticularly for  the  fainter  stars,  but  in  the  main  they  must  be 
substantially  correct.  The  bright  stars  near  the  Milky  Way 
are  only  two  or  three  times  as  numerous  as  those  near  the  poles, 
but,  as  fainter  stars  are  added,  the  ratio  increases  until  at  mag- 
nitude 17.5  the  totals  near  the  Galaxy  are  more  than  twenty 
times  those  in  the  higher  latitudes. 

TABLE  IV 

RATIOS  OF  TOTAL  NUMBERS  OF  STARS  BRIGHTER  THAN  SUCCESSIVE  UNITS 

OF  MAGNITUDE 


MAG. 

GALACTIC  LA11TUDE 

5° 

15° 

25° 

35° 

45° 

55°   j  65° 

80° 

8.5 

i 

3.2 

3.0    2.9 

2.9 

2.8 

2.8    2.7 

2.7' 

9.5 

2.8 

2.7 

2.6 

2.5 

2.5 

2.4    2.4 

2.3 

10.5 

2.8 

2.7 

2.6    2.5 

2.4 

2.4    2.3 

2.2 

11.5 

2.6 

2.5 

2.4    2.3 

2.2 

2.1     2.1 

2.0 

12.5 

. 

2.4     2.3     2.2     2.1 

2.1 

2.0     1.9    1.8 

13.5 

2.2 

2.2 

2.1 

2.0 

1.9 

1.8    1.8 

1.8 

14.5 

2.2 

2.2 

2.0 

1.9 

1.8 

1.8     1.7 

1.7 

15.5 

2.2 

2.1 

2.0 

1.8 

1.8 

1.8     1.7 

1.7 

16.5 

2.2 

2.1 

2.0 

1.8 

1.7 

1.7     1.7 

1.7 

17.5 

I 

Examining  now  the  ratios  of  adjacent  numbers  in  each 
column  of  Table  II,  which  are  analogous  to  the  corresponding 
ratios  in  Table  I  and  are  separately  listed  in  Table  IV,  we  find 
that  near  the  Milky  Way  they  are  larger  than  the  averages  for 
the  whole  sky,  while  near  the  galactic  poles  just  the  reverse 
is  true. 

The  interpretation  of  these  facts  requires  only  an  extension 
of  the  result  derived  from  Table  I.  The  numbers  of  faint  stars 
increase  much  faster  in  the  Milky  Way  than  they  do  in  higher 


THE  BRIGHTNESS  OF  THE  STARS  219 

latitudes,  but  even  in  the  Galaxy  itself  the  increase  is  far  below 
that  corresponding  to  a  uniform  distribution  throughout  space. 
The  stars  therefore  thin  out  in  all  directions,  but  much  more 
rapidly  toward  the  poles  of  the  Milky  Way  than  in  the  Milky 
Way  itself.  This  is  equivalent  to  saying  that  the  great  bulk  of 
the  stars  is  contained  in  a  much-flattened  spheroidal  region  of 
space,  whose  greatest  extension  lies  in  the  plane  of  the  Galaxy. 
In  a  general  way  these  results  have  long  been  known, 
although  certain  details,  notably  the  rapid  increase  in  the 
galactic  concentration,  have  only  lately  been  placed  beyond 
doubt.  The  conclusions  are  based  upon  simple  statistical  dis- 
cussions, but  it  should  not  be  overlooked  that  the  counts  are 
assumed  to  have  been  made  to  accurately  determined  limits  of 
brightness ;  thus  the  existence  of  a  reliable  scale  of  magnitude 
is  presupposed.  Unless  the  brightness  of  the  standard  stars 
used  for  the  determination  of  the  magnitudes  of  the  great  mass 
of  stars  is  precisely  known,  the  conclusions  will  be  vitiated  and 
rendered  uncertain  to  a  corresponding  degree.  And  herein  has 
lain  the  difficulty.  It  is  only  recently  that  the  magnitude  scale 
has  been  extended  to  the  fainter  stars  with  such  precision  as 
would  justify  confidence  in  the  results.  The  serious  obstacle 
has  been  the  enormous  range  in  the  intensity  of  the  light  of 
bright  and  faint  stars  which  had  to  be  compared  with  one 
another.  We  have  seen  that  a  range  of  20  magnitudes  cor- 
responds to  an  intensity  ratio  of  1  to  100,000,000;  that  for  17.5 
magnitudes,  the  interval  over  which  we  have  reliable  counts,  is 
1  to  10,000,000.  The  distance  from  San  Francisco  to  Chicago 
is  approximately  2,000  miles  or  about  10,000,000  feet.  The 
problem  therefore  is  analogous  to  that  of  comparing  the  length 
of  a  foot  rule  with  this  continental  distance,  but  much  more 
difficult,  for  the  percentage  error  in  measurements  of  bright- 
ness is  very  much  larger  than  that  affecting  measurements 
of  length. 

7.  THE  COLOR  OF  THE  STARS 

The  results  described  in  the  preceding  sections  are  by  no 
means  all  that  may  be  derived  from  measures  of  stellar  bright- 
ness. Thus  far  we  have  been  concerned  with  the  light  as  it 
appears  to  the  eye ;  but  starlight,  like  sunlight,  is  a  mixture  of 
many  colors,  and  it  requires  only  the  most  casual  observation 


220  THE  ADOLFO  STAHL  LECTURES 

to  learn  that  the  mixture  cannot  in  all  cases  be  the  same.  For 
example,  Vega  and  Sirius  are  white  or  bluish  white,  Capella  is 
a  golden  yellow,  while  Betelgeuze,  Antares,  and  Aldebdran 
have  various  hues  of  red.  To  produce  this  sequence  of  colors  as 
seen  by  the  eye,  the  mixtures  in  the  several  cases  must  contain 
less  and  less  of  blue  and  violet  light,  and  hence  an  increasing 
preponderance  of  red  and  yellow.  With  Antares,  for  example, 
the  excess  of  red  is  such  that  the  mixture  of  all  the  colors  radi- 
ated is  the  deep  ruby  tint  which  makes  the  star  so  conspicuous 
an  object  in  the  summer  sky. 

The  color  of  a  star,  as  we  see  it  in  the  heavens,  therefore 
depends  upon  the  relative  amounts  or  intensities  of  the  separate 
colors  which  it  radiates.  Were  it  possible  to  measure  the  sensa- 
tion of  brightness  produced  by  each  of  these,  just  as  we  have 
already  done  for  the  mixture  of  them  all,  we  should  be  able  to 
find  a  numerical  expression  for  the  color  of  the  star.  Practi- 
cally, the  conditions  are  such  that  if  we  measure  the  relative 
intensities  of  any  two  of  the  constituent  colors  which  are 
sufficiently  separated  in  the  spectral  band,  such  as  blue  and  yel- 
low, or  the  relative  amounts  of  different  groups  of  colors,  say 
of  blue  and  violet  as  compared  with  yellow  and  orange,  we 
shall  be  able  to  determine  the  resultant  color  as  it  appears  to 
the  eye.  The  extension  and  development  of  the  methods  of 
measuring  brightness  thus  suggested  must  now  be  described; 
but  first  we  may  consider  how  important  a  knowledge  of  the 
color  is  likely  to  be. 

The  color  of  the  light  radiated  from  a  luminous  source  is 
intimately  connected  with  temperature.  No  one  who  has 
watched  a  piece  of  iron  when  heated  through  all  the  shades  of 
red  to  white  heat  can  fail  to  recognize  the  closeness  of  this 
relationship.  Moreover,  with  the  stars  at  least,  temperature 
conditions  immediately  suggest  the  processes  of  growth  and 
decay,  for  it  is  improbable,  and  quite  out  of  accordance  with 
usually  accepted  ideas,  that  the  temperature  of  a  star  should 
remain  constant.  We  are  certain,  therefore,  to  obtain  from 
observations  of  color  important  information  as  to  the  physical 
condition  of  a  star  and  the  stage  of  its  development.  We  shall 
also  find  important  relations  between  the  color  of  stars  and  their 
positions  with  respect  to  the  Milky  Way,  so  that  the  adaptation 


THE  BRIGHTNESS  OF  THE  STARS  221 

of  photometric  methods  to  the  measurement  of  colors  must  also 
add  to  our  knowledge  of  the  structure  of  the  stellar  system. 

No  one  needs  now  to  be  reminded  of  the  significance  of 
observations  made  with  the  spectroscope ;  but  spectrum  analysis 
is  only  a  refined  method  of  color  analysis.  Photometric  meas- 
ures of  color,  therefore,  overlap  to  some  extent  the  field  of 
spectroscopy.  Really  they  supplement  spectroscopic  observa- 
tions, for  they  may  be  applied  to  stars  too  faint  for  examina- 
tion with  the  spectroscope. 

8.  PHOTOGRAPHIC  AND  PHOTOVISUAL  MAGNITUDES 

Since  magnitude  is  primarily  a  measure  of  physiological 
sensation,  it  depends  not  only  on  the  star  and  its  distance,  but 
also  upon  the  perceptive  peculiarities  of  the  eye.  The  light  sent 
out  by  a  star  includes  a  wide  range  of  wave-lengths  or  colors 
to  which  the  eye  is  not  equally  sensitive.  The  visual  sensibility 
is  a  maximum  in  the  yellow-orange  region  of  the  spectrum,  and 
falls  gradually  in  either  direction  toward  the  red  and  violet. 

The  relation  which  makes  an  interval  of  5  magnitudes  the 
equivalent  of  an  intensity-ratio  of  1  to  100  naturally  applies  to 
those  colors  which  rouse  the  sensation  of  luminosity.  Since,  in 
any  given  star,  these  occur  with  unequal  intensities,  the 
resultant  sensation  is  very  complex,  and,  owing  to  differences 
in  different  eyes,  cannot  be  sharply  defined.  The  measure  of 
the  resultant  visual  sensation  is  called  a  visual  magnitude,  and 
refers  to  the  normal  eye.  To  one  who  is  color  blind  the  ap- 
parent brightness  of  the  star  may  be  very  different. 

Since  a  definite  numerical  relation  connects  magnitude- 
interval  and  intensity-ratio,  magnitudes  may  be  calculated 
independently  of  any  visual  sensation,  provided  the  star's 
effective  intensity  can  be  determined.  The  photographic  plate 
provides  the  required  means  of  measuring  intensities,  and  we 
have  accordingly  systems  of  magnitudes  unrelated  to  the  eye 
and  the  measurement  of  sensation,  except  that  they  are  made  to 
agree  as  closely  as  possible  with  the  visual  scale  of  magnitudes. 

The  ordinary  photographic  plate  is  restricted  in  its  sensi- 
bility to  blue  and  violet  light.  For  all  ordinary  exposures 
the  impression  produced  by  yellow,  red,  and  orange  is  negli- 
gible. Such  a  plate  therefore  measures  mainly  the  intensity  in 


222  THE  ADOLFO  STAHL  LECTURES 

blue  and  violet,  and,  expressed  in  magnitudes  with  the  aid  of  the 
fundamental  relation,  the  result  is  called  a  photographic 
magnitude. 

Every  photographer  is  familiar  with  the  so-called  isochro- 
matic  plate.  Its  name  would  indicate  that  it  is  equally  sensitive 
to  all  colors,  but  such  is  not  the  case.  Although  affected  by 
yellow  and  orange,  it  is  far  more  sensitive  to  blue  and  violet. 
Exposed  behind  a  suitable  yellow  filter,  which  transmits  freely 
the  former  group  of  colors  but  only  slightly  the  blue  and  violet, 
it  can  be  used  to  measure  the  intensities  of  those  colors  which 
affect  the  eye.  The  combination  of  plate  and  filter  is  practically 
an  equivalent  of  the  normal  eye.  Numerically  the  resulting 
photovisual  magnitudes  are  sensibly  the  same  as  visual  magni- 
tudes, but  otherwise  have  certain  advantages  over  the  visual 
system.  They  can  be  more  reliably  and  more  rapidly 
determined,  and,  by  using  a  reflecting  telescope  and  a  standard 
brand  of  plate  and  filter,  the  results  are  sensibly  free  from  the 
kind  of  error  which  in  visual  magnitudes  arises  from  peculi- 
arities in  the  eye. 

9.     COLOR  INDEX.    THE  EXPOSURE  RATIO 

Of  the  three  kinds  of  magnitudes,  visual  and  photovisual  are 
a  measure  of  intensity  mainly  in  the  yellow  region  of  the 
spectrum,  while  photographic  magnitude  measures  the  blue 
and  violet.  A  knowledge  of  photographic  and  visual  magni- 
tudes for  the  same  star  therefore  tells  us  the  relative  amounts 
of  blue  and  yellow  light  sent  out  by  that  particular  object,  and 
hence  indicates  its  color.  For  the  actual  measurement  of  stel- 
lar colors  we  introduce  a  quantity  called  the  color  index,  defined 
by  the  equation 

Color  Index  =  Photographic  Mag.  —  Visual  Mag. 
It  is  a  matter  of  convention  as  to  the  particular  intensity 
assumed  to  correspond  to  the  zero  of  photographic  magnitude, 
and  for  convenience  it  is  determined  in  such  a  way  that  for 
white  stars  photographic  and  visual  magnitudes  are  equal. 
Such  objects  therefore  have  a  zero  color  index.  A  red  star, 
being  deficient  in  blue  and  violet  light,  is  faint  photographically, 
although  relatively  bright  in  light  to  which  the  eye  is  strongly 
sensitive.  Its  photographic  magnitude  is  therefore  numerically 


THE  BRIGHTNESS  OF  THE  STARS-  223 

greater  than  its  visual  magnitude,  and  its  color  index  is 
accordingly  a  positive  quantity,  which  for  the  reddest  stars 
amounts  to  about  two  magnitudes.  Conversely,  for  blue  stars 
the  color  index  is  negative,  but  never  very  large,  the  extreme 
value  being  about  —  0.4  magnitudes.  When  once  the  indices 
corresponding  to  known  colors  have  been  determined,  observa- 
tions of  magnitudes  afford  a  very  useful  means  of  measuring 
color. 

The  photographic  plate  can  be  used  in  quite  another  way  to 
measure  color.  The  isochromatic  plate  in  conjunction  with  the 
yellow  filter,  as  we  have  seen,  is  most  strongly  affected  by 
yellow  light,  and  may  be  said  to  produce  a  "yellow"  image. 
Without  the  filter,  its  sensitiveness  to  blue  and  violet  is 
relatively  so  great  that  the  image  is  essentially  "blue".  To 
measure  the  color  of  any  star,  we  determine  the  ratio  of  the 
exposure  times  producing  "blue"  and  "yellow"  images  of  equal 
size.  Obviously  this  ratio  must  differ  for  different  mixtures 
of  blue  and  yellow  light,  and  can  therefore  be  used  as  an  indica- 
tion of  the  star's  color.  The  result  is  called  the  exposure  ratio 
(exposure  to  blue  divided  by  exposure  to  yellow). 

It  is  not  the  purpose  of  this  account  to  deal  with  the 
numerous  applications  of  the  spectroscope  to  stellar  problems ; 
nevertheless,  to  disregard  them  altogether  would  give  an 
entirely  erroneous  impression,  for  at  many  points  spectro- 
scopic  and  photometric  methods  are  very  closely  related.  The 
earliest  measures  of  star  colors  that  we  have  were  obtained 
from  spectra  and  expressed  in  terms  of  spectral  type  or  class. 
Thus  the  familiar  notation  B,  A,  F,  G,  K,  M  signifies  not  only 
the  presence  of  certain  typical  groupings  of  lines  and  bands 
in  the  respective  spectra,  but  also  a  certain  regular  progression 
of  color,  whose  relation  to  color  index  is  given  below. 

Spectral    Class B       A      F        G       K        M 

Color    Index   -0.4    0.0    +0.4    +0.8    +1.2    +1.6 

Color  Class  b        a       f          g        k        m 

We  shall  see  presently  that  the  relation  of  color  to  spectrum 
is  not  invariable,  that  stars  with  spectra  showing  the  same 
number  and  arrangement  of  lines  may  differ  appreciably  in 
color.  For  certain  purposes  it  is  convenient  to  use  a  notation 
bearing  a  constant  relation  to  color.  The  idea  of  color  class  is 


224  THE  ADOLFO  STAHL  LECTURES 

therefore  introduced,  with  the  symbols  shown  in  the  third  line 
of  the  tabulation.  Thus  the  letters  b,  a,  f m  always  corre- 
spond to  the  color  indices  which  stand  immediately  above.  At 
the  same  time,  by  virtue  of  the  intimate  relation  between  color 
and  spectrum,  they  indicate  the  spectral  class  within 
narrow  limits. 

Although  spectral  type  is  not  an  exact  index  of  a  star's 
color,  the  spectrum  contains  information  from  which  the  color 
could  be  accurately  determined.  Nevertheless  direct  measures 
of  color,  such  as  those  given  by  the  color  index  and  the 
exposure  ratio,  are  both  convenient  and  important.  Even  for 
stars  so  bright  that  their  spectra  can  be  easily  obtained,  direct 
measures  of  color  are  more  expeditious,  while  for  the  fainter 
objects,  beyond  the  reach  of  the  spectrograph,  they  are  the 
only  methods  that  can  be  applied. 

10.     NUMBERS  AND  DISTRIBUTION  OF  STARS  OF 
DIFFERENT  COLORS 

Measures  of  color  index  and  exposure  ratio  have  only 
recently  been  undertaken.  The  data  thus  far  obtained  are 
accordingly  very  meager,  and,  for  the  most  part,  relate  to  the 
brighter  stars.  In  the  meantime,  spectral  types  have  been 
determined  for  large  numbers  of  stars,  but  these  are  necessa- 
rily restricted  to  the  brighter  objects.  The  colors  found  from 
spectra  therefore  relate  to  stars  which  are  comparatively  near, 
for  in  general,  the  brighter  stars  are  much  less  distant  than  the 
fainter  objects. 

Counts  of  about  nine  thousand  stars  from  the  Revised  Har- 
vard Photometry  give  for  the  number  of  stars  brighter  than  a 
specified  magnitude  the  totals  shown  in  Table  V5,  which  is 
similar  to  Table  I,  but  includes  no  stars  fainter  than  magni- 
tude 6.5. 

We  note  first  that  the  ratios  for  adjacent  totals  given  in 
the  right-hand  part  of  the  last  column,  for  all  the  stars 
together,  agree  sensibly  with  those  of  Table  I.  This  in  fact 
must  be  the  case,  for  the  six  principal  spectral  classes  here 
considered  include  the  great  majority  of  all  the  stars.  The 
ratios  for  the  G,  K,  M  stars  are  nearly  the  same  as  those  for 


Derived  from  Harvard  Annals,  64,   138,   140. 


N 


PLATE  XLVIIL     KAPTEYN  SELECTED  AREA  No.  40. 
a  20^  46™  5.3s;  §  +  45°  Q.5'   (1910). 

The  central  star  opposite  the  arrow  points  (at  the  right  and  at  the  bottom) 
is  8.52  magnitude. 


THE  BRIGHTNESS  OF  THE  STARS 


225 


all  classes  together;  but  for  the  blue  and  white  stars  we  find 
a  very  interesting  result.  The  numbers  for  the  B  stars  increase 
very  slowly,  while  for  the  A,  F  stars  the  totals  accumulate 
with  unusual  rapidity.  The  star-ratios  show  clearly  the 
phenomenon  in  question ;  those  for  the  B  stars  decrease  rapidly, 
and  apparently  would  become  equal  to  unity  near  magnitude 
7.5 ;  below  this  limit  the  totals  would  be  constant,  and  we 
should  conclude  that  there  are  no  B  stars  fainter  than  about 
magnitude  7.5.  These  objects  therefore  must  thin  out  very 
rapidly  with  increasing  distance. 

TABLE  V 

NUMBERS  OF  STARS  BRIGHTER  THAN  A  GIVEN  MAGNITUDE  AND  THE  STAR 
RATIO  FOR  DIFFERENT  COLORS 


MAG         B 

A,  F        G,  K,  M 

ALL 

2.5 

22 

23 

28 

73 

3.1 

2.9 

4.0 

3.4 

3.5 

68 

66 

111 

245 

2.7 

3.2 

3.5 

3.2 

4.5 

184 

211 

393 

788 

2.6 

4.4 

3.2 

3.4 

5.5 

485 

937        1264 

2686 

1.7 

4.1          3.2 

3.3 

6.5      821 

3850        4103 

8774 

For  the  A,  F  stars  an  opposite  condition  prevails.  The 
ratios  increase  and  actually  exceed  the  theoretical  maximum 
of  3.98,  which  we  have  seen  holds  for  a  uniform  distribution 
in  space;  but  they  cannot  increase  indefinitely  and,  in  fact, 
from  some  magnitude  on,- must  diminish,  otherwise  the  ratios 
in  Table  I  for  all  stars  together  could  not  decrease  as  they  do. 

About  6,100  of  the  stars  of  Table  V — those  brighter  than 
magnitude  6.25 — have  been  classified  in  Table  VI,  according 
to  spectral  type  and  position  with  respect  to  the  galactic  plane. 
The  tabular  values  are  numbers  of  stars  of  each  spectral  class 
in  regions  of  constant  area  whose  mean  distances  from  the 
Galaxy  are  given  in  the  first  column.  For  all  classes  the 
numbers  increase  with  decreasing  galactic  latitude,  but  the 
behavior  for  different  spectra  is  quite  different. 


226 


THE  ADOLFO  STAHL  LECTURES 


TABLE  VI 
SPECTRUM  AND  GALACTIC  LATITUDE — NUMBERS  OF  STARS6 


GALACTIC 

LAT 

B 

A 

1 

G 

K 

M 

ALL 

62. 

3° 

37 

296 

156 

128 

378 

101 

1096 

39. 

8 

85 

345 

152 

128 

377 

108 

1195 

21. 

6 

227 

539 

200 

170 

459 

126 

1721 

8.1 

367 

705 

212 

183 

505 

122 

2094 

SUMS 

716 

1885 

720 

609 

1719 

457 

6106 

C.AL.  CONG. 

20.0 

3.0 

1.6 

1.6 

1.5 

1.5 

2.2 

The  B  stars  show  a  high  concentration  toward  the  Galaxy, 
while  the  K  and  M  stars  are  much  more  evenly  scattered  and 
display  but  little  crowding  toward  the  Milky  Way.  The  ratios 
for  5°  and  80° — values  of  the  galactic  concentration — cannot 
be  accurately  determined  from  these  data,  but  approximately 
are  as  given  in  the  last  line  of  the  table,  ranging  from  20  for  the 
B  stars  to  only  1.5  for  the  K  and  M  stars.  The  value  of  the 
galactic  concentration  in  the  last  column  for  all  spectral 
classes  together,  namely,  2.2,  is  in  agreement  with  that  in 
Section  6  found  from  quite  different  data  (see  Table  III). 


TABLE  VII 

NUMBERS   OF    STARS   OF    DIFFERENT    COLORS    AT    0°    AND    60°    GALACTIC 
LATITUDE  AND  THEIR  AVERAGE  MOTIONS.7 


COLOR 
INDEX 

NO. 

OF    STARS 

PROPER 

MOTION 

PARALLACTIC 
MOTION 

0° 

60° 

—0.43 

45 

19 

2.4" 

3.5" 

-0.05                 230 

102                   3-1 

2.9 

+0.34                 167 

110                    7.8 

8.9 

4-0.69                 103 

47                  19.8 

20.8 

+0.96                  63 

63 

10-0 

8.6 

+  1.13 

151 

126 

7.7 

7.6 

+  1.26                 116 

83 

5.0 

4.9 

+  1.38                 106 

78 

4.1 

4.0 

+1.52 

50 

21 

4-1 

4.6 

+1.73                    7 

0        I            3.2 



6  Derived   from   Harvard  Annals,   64,    144. 
~  Derived  from  Gottinpen  Aktinomctric,  B,  34-37. 
reduced  to  the  Mount  Wilson  color  system. 


The  color  indices  have  been 


THE  BRIGHTNESS  OF  THE  STARS 


227 


The  behavior  of  the  B  stars,  the  bluest  stars  of  all,  is  very 
peculiar.  Apparently  they  comprise  a  very  limited  aggre- 
gation, closely  confined  to  the  plane  of  the  Milky  Way.  The 
concentration  of  the  A  stars  toward  the  Galaxy  is  marked,  and 
apparently  a  large  fraction  of  the  fainter  stars  in  low  galactic 
latitudes  belong  to  this  class.  The  redder  spectral  classes  are 
much  more  uniformly  distributed,  their  galactic  concentration 
being  much  below  the  average  concentration  for  all  the 
stars  together. 

Very  similar  results  are  shown  in  Table  VII,  whose  second 
and  third  columns  contain  the  numbers  of  stars  in  the  Milky 
Way  and  in  galactic  latitude  60°,  corresponding  to  the  observed 
color  indices  in  the  first  column.  The  relations  are  more  clearly 
shown  by  Fig.  17,  which  is  based  upon  the  data  of  Table  VII. 


V 


-0.4- 
b 


0.0 
a 


•1.2 

k 


FIG.  17.  VERTICAL  DISTANCES  REPRESENT  NUMBERS  OF  STARS  HAVING 
THE  COLORS  INDICATED  AT  THE  BOTTOM  OF  THE  DIAGRAM.  The  con- 
tinuous line  shows  the  relatively  larger  number  of  blue  and  white  stars 
in  the  Milky  Way  as  compared  with  the  number  in  regions  60°  distant 
therefrom,  the  latter  being  indicated  by  the  broken  line. 

Here  again  we  find  the  blue  and  white  stars  to  be  relatively 
more  numerous  in  the  Milky  Way  than  in  higher  latitudes. 
For  the  bluest  stars  the  numbers  and  curves  are  somewhat 
misleading,  since  they  do  not  include  objects  brighter  than  the 
4th  magnitude,  many  of  which  belong  to  color  class  b.  Table 
VII  is  therefore  not  altogether  comparable  with  Table  VI. 

The  thinning  out  of  the  B  stars  and  their  apparent  dis- 
appearance at  about  magnitude  7.5  raises  a  very  interesting 
question  as  to  the  behavior  of  the  faint  stars  in  relation  to  color. 
Are  there  no  blue  stars  to  be  found  among  them,  or  do  such 


228' 


THE  ADOLFO  STAHL  LECTURES 


objects  reappear  at  some  point  farther  down  the  scale  of 
magnitude;  and  what  of  the  other  color  classes?  The  results 
thus  far  obtained  are  fragmentary  but  very  suggestive. 

In  the  region  of  the  North  Pole  and  the  variable  star 
S  Cygni,  we  find  that  the  color  indices  gradually  increase  as 
we  consider  fainter  and  fainter  stars.  Among  the  brighter 
objects  we  find  zero  and  even  negative  values  of  the  index; 
but  with  decreasing  brightness  the  blue  and  white  stars 
gradually  disappear,  so  that  at  the  16th  magnitude  there  are 
no  color  indices  less  than  0.5  magnitude.  In  these  regions,  at 
least,  the  faint  stars  belong  to  the  redder  color  classes,  and  we 
find  none  bluer  than  class  /.  These  results  are  illustrated  in 
Fig.  18,  in  which  the  color  indices  are  plotted  as  vertical  dis- 
tances, opposite  the  magnitudes  of  the  individual  stars.  The 
curved  lines  along  the  lower  boundary  of  each  group  of  points 
show  the  gradual  increase  in  the  smallest  value  of  the  color 
index  occurring  among  the  stars  of  any  given  magnitude.  At 
the  16th  magnitude,  for  example,  the  bounding  curves  are 
one  square  distant  from  the  zero  line,  and  indicate,  as  already 
stated,  an  index  of  0.5  magnitude. 


FIG.  18.  HORIZONTAL  DISTANCES  ARE  PHOTOGRAPHIC  MAGNITUDES; 
VERTICAL  DISTANCES,  COLOR-INDICES.  The  smallest  value  of  the  index 
increases  with  the  magnitude.  In  the  two  regions  illustrated  there  are 
no  blue  or  white  stars  among  the  fainter  objects. 

For  the  region  of  S  Cygni  the  results  are  less  certain ;  those 
for  the  Pole  have  been  confirmed  by  observations  of  the 
exposure  ratio,  and  are  well  established.  For  faint  objects  in 
the  star  clouds  of  the  Milky  Way  determinations  of  color 
index  and  exposure  ratio  prove  that  here,  at  least,  blue  and 


THE  BRIGHTNESS  OF  THE  STARS  229 

white  stars  are  to  be  found  among  the  lower  magnitudes ;  but 
we  do  not  know  at  present  whether  they  are  confined  to  the 
Milky  Way  itself,  or  whether  there  is  a  gradual  change  in  the 
color  of  the  faint  stars  with  increasing  distance  from  the 
Galaxy,  thus  providing  a  gradual  transition  to  the  conditions 
prevailing  at  the  North  Pole  in  galactic  latitude  27°. 

Our  knowledge  of  the  distribution  of  the  various  color 
classes  over  the  face  of  the  sky  and  among  the  objects  of 
differing  brightness  is  very  much  less  than  we  should  wish, 
and  much  less  than  it  will  shortly  be.  But  we  recognize  that 
color  stands  in  close  relation  to  the  detailed  structure  of  the 
stellar  system,  and  that  the  plan  and  organization  of  the  sys- 
tem are  to  some  extent  reflected  by  the  physical  condition  of 
the  individual  stars. 

Before  turning  to  other  matters,  another  circumstance, 
clearly  shown  by  both  Tables  VI  and  VII  and  by  the  curves  of 
Fig.  17,  needs  a  word  of  comment.  The  stars  of  color  class  g 
are  seemingly  much  less  numerous  than  those  lying  just  above 
or  just  below  in  the  scale  of  color.  That  this  is  true  for  various 
parts  of  the  sky,  may  be  seen  from  the  numbers  in  each  line  of 
Table  VI,  or  from  those  in  the  second  and  third  columns  of 
Table  VII,  or,  better  still,  from  the  fact  that  both  the  curves  of 
Fig.  17  approach  the  axis  near  the  point  marked  g.  The  result 
is  well  established  for  the  stars  included  in  the  counts,  namely, 
all  those  to  a  certain  limit  of  apparent  brightness ;  but  we  must 
not  incautiously  conclude  that  it  holds  for  all  the  stars  in  the 
sky,  or  for  those  within  any  specified  distance.  The  relative 
numbers,  as  well  as  their  absolute  values,  depend  upon  the  way 
in  which  the  stars  have  been  selected.  The  question  of  selec- 
tion is  important,  and  to  understand  its  influence  in  the  present 
case,  we  must  study  the  conditions  that  determine  the  bright- 
ness of  a  star  as  we  see  it  in  the  sky. 

11.  ABSOLUTE  MAGNITUDE — A  MEASURE  OF  INTRINSIC 

BRIGHTNESS 

Aside  from  the  peculiarities  of  the  eye  or  of  the  photo- 
graphic plate,  two  factors,  distance  and  intrinsic  brightness, 
determine  the  magnitude — the  apparent  magnitude — of  a  star. 
From  the  summit  of  the  mountain  the  brightness  of  the  lights 
in  the  valley  depends  upon  their  distance  and  their  candle- 


230  THE  ADOLFO  STAHL  LECTURES 

power ;  two  lights  of  different  candle-powers  may  appear 
equally  bright  if  that  of  higher  power  is  more  distant  than  the 
other.  In  the  case  of  a  star,  to  distinguish  the  influence  of 
intrinsic  brightness  from  that  of  distance,  we  use  its  absolute 
magnitude,  a  quantity  analogous  to  candle-power;  and  just  as 
candle-power  is  a  measure  of  the  intrinsic  luminosity  of  a  light 
viewed  from  a  distance  of  one  yard,  so  is  absolute  magnitude 
an  expression  of  a  star's  intrinsic  brightness  when  seen  from  a 
standard  distance;  it  is  what  the  star's  apparent  magnitude 
would  be  were  it  viewed  from  a  distance  corresponding  to  a 
parallax  of  0.1"*.  Seen  from  this  distance  our  Sun  would  be 
near  the  limit  of  visibility,  with  an  apparent  magnitude  of  5 ; 
hence  the  Sun's  absolute  magnitude  is  5.  The  absolute  magni- 
tudes of  a  group  of  stars  therefore  express  the  range  in  their 
actual  brightness,  with  the  same  numerical  relations  between 
intrinsic  intensity  and  absolute  magnitude  that  hold  for 
apparent  intensity  and  apparent  magnitude.  A  difference  of  5 
in  the  absolute  magnitudes  means  that  the  light  of  one  star  is 
actually  one  hundred  times  more  intense  than  that  of  the  other ; 
similarly  a  difference  of  10  corresponds  to  a  ratio  in  intrinsic 
intensities  of  1  to  10,000. 

The  three  quantities — absolute  magnitude,  M,  apparent 
magnitude,  m,  and  the  distance  expressed  as  a  parallax,  jt, 
are  connected  by  a  simple  equation 

M  =  m+5  +  51ogjt.  (1) 

When  any  two  of  the  three  quantities  are  known,  the  third 
can  be  determined.  Thus  for  stars  whose  distances  and 
apparent  magnitudes  have  been  measured,  absolute  magni- 
tudes can  be  computed  from  the  formula,  just  as  it  would  be 
possible  to  calculate  the  candle-powers  of  distant  lights  in  the 
valley  if  we  knew  how  far  away  they  were  and  had  measured 
their  apparent  brightness.  Because  of  difficulties  described  in 
an  earlier  section,  the  distances  of  only  a  few  objects  have  been 
directly  measured,  and  these  alone  tell  us  little  of  the  real 
brightness  of  the  stars. 

Fortunately,  as  has  been  shown  by  Adams  and  Kohlschiitter, 
it  is  possible  to  find  the  absolute  magnitude  directly  from  the 

*  This  is  equivalent  to  saying  that  the  radius  of  the  Earth's  orbit  seen  from 
the  standard  distance  would  subtend  an  angle  of  0.1".  A  length  of  one  foot  at  a 
distance  of  400  miles  subtends  the  same  angle.  The  value  of  the  standard  dis- 
tance commonly  used,  in  light-years,  is  33. 


THE  BRIGHTNESS  OF  THE  STARS 


231 


spectrum,  at  least  for  all  but  the  bluest  spectral  types.  The 
relative  intensities  of  certain  pairs  of  lines,  even  in  spectra  of 
the  same  class,  vary  with  the  intrinsic  brightness  of  the  star; 
and  by  observing  these  critical  lines,  the  absolute  magnitude  is 
quickly  and  accurately  determined. 

—  2        0+2+4+6      +8    +10  +12 


Fo-F9 


Go-G9 


Ko-K3 


M 


A 


FIG.  19.  VERTICAL  DISTANCES  REPRESENT  NUMBERS  OF  STARS  HAVING 
THE  ABSOLUTE  MAGNITUDES  GIVEN  AT  THE  TOP  OF  THE  DIAGRAM, 
different  spectral  classes  being  shown  separately.  The  grouping  of 
the  stars  as  giants  and  dwarfs  is  clearly  indicated,  the  former  having 
an  absolute  magnitude  of  about  +1,  while  the  magnitude  of  the  dwarfs 
increases  as  the  M  stars  are  approached.  The  curves  are  from  the 
investigation  by  Adams  and  Joy,  Mt.  Wilson  Contr.,  No.  142;  Astro- 
physical  Journal,  46,  335,  1917. 

The  classification  and  study  of  such  absolute  magnitudes 
as  are  now  available  lead  to  a  very  remarkable  result.  The 
bluest  stars,  on  the  average,  are  about  one  hundred  times  more 
luminous  than  the  Sun.  Their  mean  absolute  magnitude  is  not 
far  from  zero,  and  the  individuals  differ  but  little  from  the 
mean.  A  similar  result  holds  for  the  A  stars,  but  with  a  wider 


232  THE  ADOLFO  STAHL  LECTURES 

range  in  the  individual  values.  For  the  redder  spectral  classes 
the  behavior  of  the  absolute  magnitudes  is  that  shown  by 
Fig.  19,  in  which  vertical  distances  represent  numbers  of  stars 
having  the  absolute  magnitudes  shown  at  the  top.  With 
increasing  spectrum  or  color  the  range  of  intrinsic  brightness 
increases  until  for  the  M  stars  we  find  objects  as  bright  as 
magnitude  — 3  and  as  faint  as  +13.  Beginning  with  F  there 
are  for  each  type  two  values  of  the  absolute  luminosity  which 
occur  more  frequently  than  any  others,  as  is  shown  by  the 
presence  of  two  maxima  in  each  of  the  curves.  One  of  these 
is  always  near  absolute  magnitude  +1,  while  the  other  has  the 
gradually  increasing  values,  +4,  +5,  +6,  +8,  and  +11  for 
each  of  the  successive  spectral  intervals  illustrated  in  the 
figure.  For  example,  the  two  groups  of  G  stars  differ  in  their 
mean  absolute  brightness  by  nearly  five  magnitudes.  For  the 
M  stars  the  difference  is  nearly  10  magnitudes,  corresponding 
to  a  ratio  of  1  to  10,000  in  the  intensities.  In  this  case  the  two 
groups  are  clearly  separated  by  an  interval  of  about  6  magni- 
tudes within  which  there  are  no  M  stars  whatever. 

Were  we  to  measure  the  candle-power  of  a  large  number 
of  the  lights  in  the  valley,  we  should  find  a  similar  result ;  the 
arc  lamps  used  for  street  illumination  would  be  separated  by 
a  wide  interval  of  brightness  from  the  incandescent  bulbs  of 
100  candle-power  or  more.  The  illustration  here  is  more  or 
less  trivial,  but  it  would  seem  less  so  had  we  no  previous 
knowledge  of  the  practice  in  illumination  and  were  we,  as  in 
the  case  of  the  stars,  unable  to  estimate  approximately  with 
the  unaided  eye  the  relative  distances  of  the  lights  observed. 

The  very  extraordinary  splitting  up  of  the  redder  spectral 
types  into  two  sharply  marked  subdivisions  has  led  to  the 
introduction  of  the  terms  "giant"  and  "dwarf".  It  is  remark- 
able and  undoubtedly  a  significant  fact  in  the  history  of  a  star's 
development  that  objects  having  similar  spectra  should  differ 
so  greatly  in  absolute  luminosity  as  do  the  giants  and  dwarfs. 
The  similarity  of  spectrum  and  color  means  that  the  surface 
temperature,  and  hence  the  amount  of  light  radiated  from  a 
constant  area  of  the  surface,  is  approximately  the  same  for 
both  classes  of  stars;  hence  the  range  of  1  to  10,000  in  the 
average  absolute  luminosities  of  giant  and  dwarf  M  stars  must 


w 


PLATE  XLIX.     STAR  CLOUDS  AND  VACANT  LANES   NEAR  @  OPHIUCHI. 


From  a  photograph  with  the  Bruce  telescope,  by  E.  E.  Barnard. 


THE  BRIGHTNESS  OF  THE  STARS  233 

be  attributed  to  differences  in  their  linear  dimensions.  To 
radiate  more  light,  the  giants  must  be  larger  than  the  dwarfs ; 
and  that  their  surfaces  may  be  in  the  required  ratio  of  10,000 
to  1,  the  diameters  of  the  giants  must  be  a  hundred  times  those 
of  the  dwarfs. 

But  note  what  this  implies — a  ratio  in  the  volumes  of 
1,000,000  to  1.  What  are  we  to  infer  as  to  the  masses  and 
densities  of  these  stars?  Without  additional  evidence  the 
question  cannot  be  answered;  but  we  may  make  two  extreme 
assumptions:  (a)  The  densities  of  giant  and  dwarf  stars  are 
the  same;  the  masses  of  the  former  will  then  be  a  million 
times  those  of  the  latter,  (b)  The  masses  are  equal;  the 
density  of  the  giants  can  then  be  only  one-millionth  that  of  the 
dwarfs.  The  available  evidence  indicates  that  the  second  sup- 
position is  much  nearer  the  truth  than  the  former,  and  that  the 
great  difference  in  volume  between  giant  and  dwarf  stars  is 
to  be  explained  by  differences  in  attenuation  of  the  material  of 
which  they  are  composed  rather  than  by  differences  in  the 
amount  of  that  material. 

The  critical  factor  underlying  these  conclusions  as  to  the 
linear  dimensions  of  giant  and  dwarf  stars  is  emphasized  by 
the  fact  that  they  do  not  hold  for  the  electric  arcs  and 
incandescent  bulbs — the  giants  and  dwarfs  among  the  lights  of 
the  valley.  The  stars  compared  have  the  same  general  type  of 
spectrum,  and  hence  nearly  the  same  surface  temperatures; 
but  the  lights  represent  wide  differences  in  temperature,  and, 
from  candle-power  alone,  we  can  conclude  nothing  as  to  the 
extent  of  the  luminous  surface  emitting  the  light  seen  by  an 
observer  on  the  mountain. 

12.    RELATION  OF  COLOR  TO  ABSOLUTE  MAGNITUDE 

We  have  referred  to  the  fact  that  the  spectrum  of  a  star  is 
not  an  exact  measure  of  its  color,  and  that  objects  having  the 
same  type  of  spectrum  may  show  appreciable  differences  in 
color.  Investigations  by  several  observers  have  shown  that 
these  differences  are  related  to  the  absolute  magnitudes  of  the 
stars,  the  more  luminous  objects  being  the  redder. 

Determinations  of  the  exposure  ratio  for  a  group  of  giant 
and  dwarf  stars  illustrate  the  nature  of  the  dependence.  The 


234 


THE  ADOLFO  STAHL  LECTURES 


results  are  illustrated  in  Fig.  20,  in  which  vertical  distances 
represent  the  logarithm  of  the  exposure  ratio,  while  horizontal 
distances  correspond  to  spectral  types.  The  circles  indicate 
giant  stars  and  the  points  dwarfs.  The  dotted  line  gives  the 
variation  in  the  logarithm  of  the  exposure  ratio  for  the  colors 


9.6 


9.4 


9.0 


V 


FIG.  20.  VARIATION  OF  COLOR  WITH  SPECTRUM  FOR  GIANT  (HEAVY 
LINE)  AND  DWARF  (LIGHT  LINE)  STARS.  Vertical  distances  represent 
logarithms  of  the  ratio  of  exposure  for  blue  light  to  exposure  for 
yellow  light  necessary  to  produce  the  same  photographic  effect.  For 
stars  having  G  and  K  spectra  the  giants  are  appreciably  redder  than 
the  dwarfs. 

normally  assumed,  in  the  Mount  Wilson  color  system,  to 
correspond  to  the  different  spectral  types,  and  was  derived 
from  the  color  indices  and  spectra  of  the  Polar  Standards  of 
magnitude.  None  of  the  giants  differs  greatly  from  zero  abso- 
lute magnitude,  and  the  progession  of  exposure  ratio  with 
spectrum  for  these  stars  is  fairly  regular.  The  dwarf  stars 
average  4  or  5  magnitudes  fainter  than  the  giants,  and 
although  the  scattering  of  the  points  is  considerable,  the 
change  in  the  exposure  ratio  with  spectral  class,  and  the 
relation  of  color  to  absolute  brightness  are  clearly  enough 
shown. 

Since  the  ratio  is :  exposure  to  blue  divided  £3;  exposure  to 
yellow,  a  large  value  of  the  ratio  implies  a  deficiency  of  blue 
light,  and  hence  an  excess  of  red  light.  The  giant  stars  are 
clearly  redder  than  the  dwarfs,  as  already  stated,  the  difference, 
expressed  in  color  index,  easily  amounting  to  half  a  magnitude. 
For  the  late  A  and  early  F  spectra  the  color  difference  is 


THE  BRIGHTNESS  OF  THE  STARS  235 

inappreciable,  and  the  diagram  suggests  that  further  observa- 
tions may  show  that  the  curves  for  giants  and  dwarfs  cross  at 
this  point,  thus  giving  a  reversal  of  the  effect  for  the  B  stars. 

13.  RELATION  BETWEEN  THE  DISTANCES  OF  STARS  AND  THEIR 
APPARENT  MOTIONS 

In  an  earlier  section  we  have  seen  something  of  the 
difficulties  encountered  in  attempting  to  measure  directly  the 
distances  of  the  stars.  A  more  expeditious  method  is  to  make 
use  of  equation  (1),  by  means  of  which  the  parallax,  Jt,  can  be 
calculated  when  the  absolute  and  apparent  magnitudes  have 
been  determined.  Intrinsic  brightness,  as  we  have  seen,  can  be 
derived  from  the  spectrum  for  all  but  the  bluer  spectral  classes, 
and  apparent  magnitude  can  be  measured  by  the  method  out- 
lined in  Section  3. 

This  is  the  most  valuable  method  that  we  possess  for  the 
determination  of  the  distances  of  large  numbers  of  individual 
stars;  but  it  has  only  recently  been  developed,  and  in  the 
meantime  relations  between  the  distance  and  apparent  motions 
of  the  stars  give  valuable  information  as  to  the  average 
distance  of  any  particular  class  of  stars,  say  those  of  the  sixth 
apparent  magnitude,  or  those  having  G-type  spectra. 

The  principles  involved  are  simple.  Suppose  we  observe 
from  the  mountain,  not  the  lamps  that  light  the  streets  of  a 
distant  town,  but  those  of  the  moving  motor  cars  within  its 
limits.  These  will  be  traveling  here  and  there  in  all  directions, 
with  a  considerable  range  of  speed.  During  a  given  interval, 
say  one  minute,  the  direction  in  which  each  car  is  seen  will 
change  a  certain  amount.  The  average  of  all  these  changes 
in  direction  depends  upon  the  average  speed,  in  miles  per 
hour,  with  which  the  cars  are  moving.  Suppose  that  the 
average  change  in  direction  has  also  been  determined  for  the 
motors  of  a  second  town,  and  assume,  further,  that  the 
average  speed  per  hour  is  in  both  places  the  same.  The 
relative  distances  of  the  two  towns  from  the  observer  can  then 
be  found.  If  the  average  change  in  the  positions  of  the  moving 
cars  is  the  same,  the  towns  are  equally  distant;  and  if  one 
average  is  smaller  than  the  other,  the  town  to  which  it 
corresponds  is  the  more  distant  of  the  two. 


236  THE  ADOLFO  STAHL  LECTURES 

The  total  annual  change  in  the  direction  in  which  a  star  is 
seen  is  called  its  proper  motion.  If  we  suppose  that  the 
average  speed  of  the  stars,  in  miles  per  second,  is  everywhere 
the  same,  it  follows  that  the  average  proper  motion  of  distant 
stars  will  be  smaller  than  that  of  nearer  objects;  and  by  com- 
paring the  average  motions  of  different  groups  of  stars  we  can 
find  their  relative  distances.  Finally,  from  the  motions  of 
stars  whose  distances  have  been  directly  measured,  we  derive 
a  relation  between  average  proper  motion  and  parallax  which 
can  be  used  to  find  the  average  distance  of  any  group  of  stars 
whose  motions  have  been  observed. 

By  way  of  illustration,  there  is  given  in  the  fourth  column 
of  Table  VII  the  average  proper  motion  during  an  interval 
of  a  hundred  years,  for  each  of  the  groups  of  stars  whose  mean 
color  indices  appear  in  the  first  column  of  the  table.  These 
numbers  increase  to  a  maximum  and  then  decline  again ;  and 
from  them  we  infer  that  the  stars  showing  the  extremes  of 
color  are  the  most  distant,  while  those  of  the  intermediate  color 
classes,  with  indices  of  about  -J-0.7,  are  nearest  to  us. 

There  is  another  and  even  more  important  method  of  using 
the  apparent  motions  of  the  stars  to  find  the  average  distance 
of  any  class  of  objects.  Suppose  the  observer  on  the  mountain 
to  walk  along  its  top,  as  in  an  earlier  illustration — toward  the 
west,  we  assume,  and  with  a  known  rate  of  motion.  During  a 
minute  he  will  have  moved  say  a  hundred  yards.  In  the  mean- 
time the  lights  in  the  valley  to  the  south  will  apparently  have 
shifted  toward  the  east  by  a  definite  angular  amount,  whose 
value  can  be  found  by  measurement.  The  problem  of 
determining  the  distance  of  the  lights  is  just  that  of  calculating 
the  distance  from  which  a  length  of  one  hundred  yards  sub- 
tends an  angle  equal  to  the  observed  change  in  direction.  If 
the  lights  are  not  in  motion,  observation  of  any  one  of  them 
determines  the  distance  of  the  town  to  which  it  belongs. 

But  suppose  that  we  again  observe  the  lights  of  the  moving 
motor  cars  within  the  town.  During  the  minute  in  which  the 
observer  walks  the  hundred  yards,  each  motor  moves  a  certain 
distance.  Apparent  changes  in  direction  are  produced  by  the 
observer's  motion  as  before,  but  these  are  modified  by  the 
motions  of  the  cars  themselves.  For  some  the  eastward  dis- 
placement is  increased,  for  others,  diminished ;  but  for  a  large 


THE  BRIGHTNESS  OF  THE  STARS  237 

number  of  cars,  moving-  at  random,  and  with  random  speeds, 
the  individual  motions  compensate  each  other  and  the  average 
displacement  toward  the  east  is  the  same  as  though  the  cars 
were  all  at  rest.  The  distance  of  the  town  can  therefore  be 
calculated  as  accurately  as  before. 

And  thus  we  can  compute  the  average  distance  of  a  certain 
group  or  class  of  stars  when  their  individual  motions  are  at 
random.  The  Sun  and  its  attendant  planets,  moving  through 
space  in  a  definite  direction  with  known  speed,  carry  with  them 
the  observer  who,  after  an  interval,  measures  the  changes  in 
direction  of  the  stars.  Their  apparent  motions  are  the  result 
of  the  observer's  change  in  position,  combined  with  the  motions 
of  the  stars  themselves. 

Each  individual  proper  motion  is  analyzed  into  two  com- 
ponents, one  parallel  to  the  motion  of  the  Sun,  the  other 
perpendicular  to  this  motion.  The  latter  must  be  due  entirely 
to  the  real  motion  of  the  star,  but  the  former — the  parallactic 
component  as  it  is  called — is  produced  partly  by  the  motion  of 
the  star  and  partly  by  the  motion  of  the  Sun.  For  objects 
moving  at  random,  the  part  due  to  the  real  motions  of  the  stars 
will  vary  in  amount,  and  sometimes  will  be  in  the  same  direction 
as  the  solar  motion  and  sometimes  opposite  thereto.  If,  there- 
fore, we  form  the  average  of  the  parallactic  components  for  a 
large  number  of  stars,  their  individual  motions  will  compensate 
each  other  and  the  mean  will  be  the  same  as  though  the  stars 
were  all  at  rest.  The  result,  which  represents  the  effect  of  the 
observer's  motion  upon  the  direction  in  which  the  stars  are 
seen,  is  the  parallactic  drift  or  motion  of  the  group  of  stars 
observed,  and  corresponds  to  the  eastward  dispflacement  of 
the  lights  produced  by  the  change  in  the  position  of  the  observer 
on  the  mountain  top. 

The  parallactic  drift,  combined  with  the  known  motion  of 
the  Sun,  gives  at  once  the  mean  distance  of  the  group  of  stars 
observed.  The  method  is  not  applicable  to  stars  directly  in 
the  line  of  the  observer's  motion,  but  gives  useful  results  for 
objects  in  other  parts  of  the  sky.  The  fact  that  he  is  in  the 
midst  of  the  stellar  system,  with  stars  on  every  hand,  does  not 
alter  the  problem  essentially. 

The  mean  parallactic  drift  for  an  interval  of  a  hundred 
years  is  also  given  in  Table  VII  for  stars  of  different  color 


238  THE  ADOLFO  STAHL  LECTURES 

index.  The  numbers  vary  inversely  as  the  distances,  and  con- 
firm our  earlier  conclusion  as  to  the  relative  distance  of  the 
stars  of  different  color.  The  /  and  g  stars  are  nearest  to  us, 
and,  occupying  a  smaller  volume  of  space  than  the  other  color 
classes,  we  should  expect  them  to  be  less  numerous.  This 
perhaps  accounts  for  part  of  the  deficiency  in  the  numbers  of 
these  objects,  to  which  reference  has  already  been  made.  The 
matter  is  not  altogether  clear,  however,  for  the  smaller 
distances  of  these  stars  indicate  that  their  mean  absolute 
luminosities  are  below  the  average  of  the  other  color  classes. 
The  parallactic  motions  of  the  very  red  stars  are  of  the  same 
order  of  magnitude  as  those  of  the  blue  stars.  The  mean 
distances  and  luminosities  of  both  these  classes  of  stars  must 
therefore  be  sensibly  the  same.  By  consulting  Fig.  19  we  see 
that  this  result  apparently  can  be  brought  about  only  by  exclud- 
ing the  dwarf  K  and  M  stars.  It  is  probable,  therefore,  that 
our  counts  contain  none  of  the  dwarfs  of  these  spectral  classes. 
The  F  and  G  dwarfs,  however,  being  appreciably  brighter,  may 
fall  within  the  limit  of  apparent  magnitude  used  in  the  selection 
of  the  data.  This  would  account  for  the  relatively  low  average 
luminosity  of  the  F  and  G  stars,  but,  on  the  other  hand,  would 
seem  to  indicate  that  the  manner  of  selection  had  introduced  a 
larger  percentage  of  F  and  G  stars  than  of  K's  and  M's,  i.e., 
giant  stars  plus  some  dwarfs,  whereas,  of  the  others,  we  have 
only  giants.  The  actual  number  of  the  F's  and  G's  included, 
as  we  have  seen,  is  comparatively  small  and  may  therefore 
indicate  a  real  deficiency  for  these  spectral  classes.  These 
details  show  the  artificial  character  of  apparent  magnitude  as 
a  limit  in  choosing  data,  and  illustrate  some  of  the  disturbing 
effects  of  selection  referred  to  at  the  end  of  Section  10. 

14.    RELATION  OF  ABSOLUTE  MAGNITUDE  TO  VELOCITY  OF 

MOTION 

Recent  accumulations  of  data  bearing  on  the  intrinsic 
brightness  of  the  stars  have  brought  to  light  a  very  significant 
relation  between  the  absolute  magnitude  of  a  star  and  the 
speed  with  which  it  moves  through  space.  The  nature  of  the 
relation  is  illustrated  by  Fig.  21,  in  which  vertical  distances 
represent  absolute  magnitudes,  and  horizontal  distances  the 
speed  in  kilometers  per  second  with  which  the  star  is  moving. 


THE  BRIGHTNESS  OF  THE  STARS 


239 


The  figure  summarizes  the  results  by  Adams  and  Stromberg 
from  about  1,300  stars,  which  were  divided  into  two  groups, 
one  including  F  and  G  spectra  (points),  the  other  K  and  M 
spectra  (crosses).  For  both  groups  there  is  a  regular  and 
sensibly  linear  increase  in  average  radial  velocity  with 
increasing  absolute  magnitude.  The  gain  in  velocity  amounts 
to  about  1.5  kilometers  per  second  for  each  unit  of  magnitude, 
and  applies  to  both  giants  and  dwarfs.  The  gap  between  the 
K  and  M  stars  of  high  and  low  luminosity  is  clearly  shown 
between  the  upper  two  and  the  lower  three  crosses  of  the 
diagram.  For  the  F's  and  G's,  as  shown  by  Fig.  19,  the  giants 
and  dwarfs  are  not  entirely  separated,  so  that  points  in  Fig.  21 
are  more  uniformly  spaced. 


17  /g  /$  zo  si  zz  zi  *>  zs  &  27  &  &  x 


FIG.  21.  VARIATION  OF  RADIAL  VELOCITY  (HORIZONTAL  DISTANCES) 
WITH  ABSOLUTE  MAGNITUDE  (VERTICAL  DISTANCES).  Points  represent 
groups  of  stars  having  F  and  G  spectra;  crosses  represent  similar 
groups  of  K  and  M  stars.  From  the  investigation  by  Adams  and 
Stromberg,  Mt.  Wilson  Contr.,  No.  131 ;  Astro  physical  Journal,  45,  293, 
1917. 

Finally,  it  will  be  noted  that  for  the  same  absolute  magni- 
tude, the  K  and  M  stars  appear  to  be  moving  with  higher 
average  speeds  than  the  F's  and  G's. 

The  explanation  of  these  relations  and  their  significance  as 
a  mechanical  feature  of  the  stellar  universe  are  not  at  present 
known.  They  may  depend  upon  the  masses  of  the  stars,  the 
smaller  objects  moving  faster,  on  the  average,  than  those  of 
greater  mass.  On  the  other  hand,  the  dwarfs  may  represent 
a  later  stage  in  the  development  which  we  commonly  suppose 
each  star  to  undergo,  and  there  may  be  circumstances  which 


240  THE  ADOLFO  STAHL  LECTURES 

cause  a  gradual  acceleration  of  motion  during  the  progress  of 
the  star's  development.  These  are  only  suggestions;  a  satis- 
factory explanation  must  await  the  accumulation  of  further 
data. 

15.    THE  SYSTEMATIC  MOTIONS  OF  THE  STARS 

For  two  centuries  we  have  known  that  the  stars  have 
motions  of  their  own,  but  for  only  a  short  time  has  it  been 
clear  that  they  do  not  move  at  random.  We  now  know  many 
groups  of  stars  which  seem  to  be  definitely  organized  physical 
systems,  whose  members  travel  through  space  along  parallel 
paths  at  a  constant  speed.  The  bright  stars  of  Ursa  Major,  the 
Pleiades,  a  part  of  the  Constellation  of  Taurus,  a  cluster  in 
Perseus  and  one  in  Scorpio,  and  the  B-type  stars  in  the  vicinity 
of  Orion,  not  to  mention  many  smaller  aggregations,  move  as 
groups  and  thereby  suggest  that,  besides  their  community  of 
motion,  they  possess  other  characteristics  in  common.  But 
these  moving  clusters  comprise  only  a  minute  fraction  of  the 
total  number  of  stars,  and  apparently  have  no  close  relation 
to  the  two  great  streams  which  appear  to  be  one  of  the  chief 
characteristics  of  the  organization  of  our  stellar  system.  The 
phenomenon  of  stream  motion,  which  now  requires  our 
attention,  is  probably  as  significant  a  factor  for  stellar  move- 
ments, as  is  the  crowding  of  the  stars  in  the  Milky  Way  for 
the  form  of  the  stellar  universe. 

Until  1904  it  was  commonly  assumed,  as  has  been  done  in 
the  preceding  sections,  that  the  vast  majority  of  the  stars 
might  be  regarded  as  moving  at  random.  Kapteyn,  however, 
has  shown  that  this  is  not  even  approximately  the  truth.  The 
facts  of  the  case  can  be  learned  by  a  study  of  proper 
motions ;  but  these  must  be  known  with  precision  for  a  large 
number  of  stars  well  scattered  over  the  sky. 

The  principle  underlying  the  analysis  is  not  difficult  to 
understand.  For  a  chosen  region  of  the  sky,  with  not  too 
great  an  area,  we  count  the  numbers  of  proper  motions  having 
definite  directions  on  the  surface  of  the  celestial  sphere;  we 
find  a  certain  number  toward  the  north,  so  many  directed  10° 
east  of  north,  so  many  20°  east  of  north,  and,  similarly,  on 
around  the  circuit  of  360°.  Now  let  us  construct  a  diagram 
with  lines  radiating  from  a  central  point  at  intervals  of  10°, 


THE  BRIGHTNESS  OF  THE  STARS  241 

the  length  of  each  line  being  proportional  to  the  number  of 
proper  motions  in  one  of  the  specified  directions;  the  ends  of 
the  radiating  lines  are  then  connected  by  a  closed  curve,  and 
the  result  is  called  a  velocity  diagram. 

For  stars  moving  at  random,  and  a  solar  system  fixed  with 
respect  to  the  center  of  gravity  of  the  system  of  the  stars,  the 
proper  motions  will  be  equally  numerous  in  all  directions ;  the 
radii  which  represent  them  will  be  equal>  and  the  velocity 
diagram  will  be  a  circle.  If  we  still  suppose  random  motions 
for  the  stars,  but  assume  the  observer  to  be  in  motion,  the 
velocity  diagram  becomes  an  oval  with  the  point  of  origin  for 
the  radii  no  longer  at  the  middle  of  the  figure.  The  elongation 
of  the  oval,  in  direction  and  amount,  and  the  position  of  the 
origin  of  the  radii  depend  upon  the  observer's  motion. 

The  result  of  analyzing  the  proper  motions  actually 
observed  varies  with  the  region  of  the  sky  considered,  but  is 
always  a  velocity  diagram  differing  in  a  very  characteristic 
way  from  those  described  above.  The  diagrams  are  no 
longer  simple  ovals,  but  usually  pear-shaped  figures,  which 
can  be  accounted  for  only  by  supposing  that  the  proper 
motions  have  a  marked  preference  for  two  certain  directions. 
A  comparison  of  these  preferential  directions,  which  can  be 
determined  from  the  velocity  diagrams  of  different  regions, 
shows  that  they  fall  into  two  groups,  and  that  the  directions 
of  each  group  converge  and  practically  intersect  in  a  single 
point  called  the  apex. 

Kapteyn  showed  that  the  phenomena  are  satisfactorily 
explained  by  supposing  that  the  great  majority  of  the  stars 
belong  to  one  or  the  other  of  the  two  great  interpenetrating 
swarms  whose  motions,  relative  to  the  solar  system,  make  with 
each  other  an  angle  of  about  100°.  The  speeds  are  as  1.52  to 
0.86,  and  the  numbers  of  stars  in  each  stream  are  as  3  to  2. 
The  fundamental  nature  of  the  phenomenon  is  indicated  by 
the  fact  that  the  motion  of  one  swarm  relative  to  the  other  is 
almost  exactly  parallel  to  the  plane  of  the  Milky  Way.  It  is 
not  to  be  supposed,  however,  that  the  hypothesis  of  two 
streams  of  stars  is  more  than  a  first  approximation  to  the 
systematic  motions  of  the  stellar  system. 

The  question  of  systematic  motions  has  also  been  investi- 
gated on  the  basis  of  velocities  in  the  line  of  sight 'determined 


242  THE  ADOLFO  STAHL  LECTURES 

with  the  spectrograph.  Here  we  use,  not  the  number  of 
motions,  but  the  average  value  of  the  line-of-sight  or  radial 
velocity  for  the  stars  in  each  part  of  the  sky.  The  results  may 
be  represented  graphically  in  a  manner  similar  to  that  used 
in  preparing  the  velocity  diagrams  described  above,  except 
that  now  the  radiating  lines  are  not  confined  to  a  plane,  but 
diverge  in  all  directions  into  space,  one  for  the  direction  of 
each  region  of  the-  sky  for  which  a  group  of  radial  velocities 
has  been  determined.  The  length  of  each  radiating  line  is 
made  proportional  to  the  mean  radial  velocity  of  the  stars 
selected  in  that  direction,  and  through  the  extremities  of  them 
all  is  passed  a  closed  surface  called  the  velocity  surface.  The 
variation  in  the  distance  of  this  surface  from  the  point  of 
origin  within  represents  the  variation  in  the  average  radial 
velocity  from  point  to  point  in  the  sky. 

In  a  recent  investigation  by  Stromberg,  the  data  which 
included  stars  of  F,  G,  and  K  spectral  types  were  divided  into 
three  groups  according  to  luminosity,  with  mean  absolute 
magnitudes  of  approximately  1,  2,  and  6.  The  groups  were 
discussed  separately  with  results  which  are  represented  in 
Figs.  22,  23,  and  24.  The  points  in  the  three  diagrams  lie  in  the 
plane  of  the  Milky  Way.  The  full-line  curves  are  intersec- 
tions of  the  galactic  plane  with  the  smooth  velocity  surface 
best  representing  the  observed  average  velocities.  The  dotted 
curves  are  similar  intersections  with  the  best-fitting  sym- 
metrical surfaces. 

It  will  be  noted  first  that  the  linear  dimensions  of  the 
figure  for  the  most  luminous  stars  are  smallest,  and  largest 
for  that  corresponding  to  the  faintest  stars.  This  agrees 
perfectly  with  the  relation  between  absolute  luminosity  and 
speed  found  in  the  preceding  section.  Next  it  will  be  seen 
that  the  general  characteristics  of  the  three  figures  are  the 
same.  The  symmetrical  curves  all  have  their  longest  axes  in 
longitudes  near  170° ;  in  this  direction,  which  agrees  well  with 
that  of  the  stream  motion,  the  average  radial  velocity  is 
highest. 

The  curves  corresponding  to  the  velocity  surfaces  which 
more  accurately  represent  the  data  are  three-lobed,  and  the 
preferential  directions  of  highest  velocity  no  longer  form  a 
straight  line,  but  are  inclined  at  an  angle  which  is  smallest 


THE  BRIGHTNESS  OF  THE  STARS 


243 


9(T 


FIG.  22,  23,  24.  INTERSECTIONS  BETWEEN  THE  AVERAGE  RADIAL- 
VELOCITY  SURFACES  AND  THE  GALACTIC  EQUATOR  FOR  THREE  GROUPS 
OF  STARS:  Fig.  22,  for  the  stars  intrinsically  brightest  and  most  dis- 
tant; Fig.  24,  for  those  intrinsically  faintest  and  nearest;  Fig.  23,  for 
the  stars  intermediate  in  luminosity  and  distance.  The  distances 
of  the  points  from  the  intersection  of  the  reference  lines  represent 
the  average  radial  velocity  in  regions  near  the  galactic  equator.  The 
projections  of  the  longest  axes  of  the  surfaces  are  indicated  by  straight 
lines ;  the  arrows  indicate  the  position  of  approximate  planes  of  sym- 
metry perpendicular  to  the  galactic  equator.  From  the  investigation  by 
Stromberg,  Mt.  Wilson  Contr.,  No.  144;  Astrophysical  Journal,  47,  7, 
1918. 


244  THE  ADOLFO  STAHL  LECTURES 

for  the  most  distant  stars.  The  arrows  directed  downward 
bisect  these  angles  approximately,  and  indicate  what,  for  other 
reasons,  we  believe  to  be  the  direction  of  the  center  of  the 
stellar  system.  The  axes  of  greatest  mobility,  therefore,  seem 
to  coincide  better  with  a  curve  than  with  a  straight  line,  and 
the  directions  of  the  preferential  velocities  are  such  as  might 
be  expected  from  a  general  circulation  of  the  stars  about  the 
center  of  the  system,  with  a  strong  tendency  toward  motion  in 
the  galactic  plane ;  the  data  possibly  indicate  that  something  of 
the  sort  is  taking  place. 

From  the  radial  motions  we  have  determined  the  directions 
of  the  highest  average  velocity,  while  from  the  proper  motions 
we  have  found  the  line  in  space  along  which  motions  most 
frequently  occur  irrespective  of  their  size.  Although  we 
might  expect  the  resulting  directions  to  coincide,  the  things 
investigated  are  quite  distinct.  The  radial  .velocities  confirm, 
in  a  general  way,  the  existence  of  the  two  star  streams,  but 
at  the  same  time  suggest  a  modification  of  this  explanation  of 
the  systematic  motions  of  the  stars,  which  may  ultimately 
throw  much  light  on  the  structure  and  mechanics  of  the 
universe. 

16.  SUMMARY 

The  main  part  of  the  preceding  account  deals  with  the 
brightness  of  the  stars  and  with  numerous  questions  connected 
with  the  determination  of  magnitudes.  Simple  counts  of 
stars,  if  made  to  specified  limits  of  a  precisely  determined 
scale  of  magnitudes,  give  much  information  about  the  form 
and  extent  of  the  stellar  system,  which  appears  to  be  a  great 
flattened  cluster,  many  thousand  light-years  in  diameter,  with 
the  Milky  Way  as  a  structural  feature  of  first  importance. 

Observing  the  colors  of  the  stars,  either  with  the  spectro- 
graph,  or  by  comparing  their  visual  and  photographic  magni- 
tudes, we  learn  that  objects  in  different  physical  states  are  not 
scattered  at  random  throughout  space,  but  show  a  charac- 
teristic arrangement  with  respect  to  the  galactic  plane. 

From  determinations  of  stellar  distance  and  apparent 
magnitude,  we  find  that  the  stars  display  an  extraordinary 
range  of  intrinsic  brightness.  Occasional  objects  are  10,000 
times  as  luminous  as  our  Sun,  while,  at  the  other  extreme, 


THE  BRIGHTNESS  OF  THE  STARS  245 

there  are  probably  stars  with  Only  1/10,000  part  of  the  solar 
luminosity. 

All  the  blue  and  white  stars,  intrinsically,  are  intensely 
bright,  but  for  the  redder  color  classes  we  find  the  remarkable 
subdivision  into  giants  and  dwarfs,  with  wide  differences,  not 
only  in  absolute  magnitude,  but  perhaps  also  in  density. 

We  also  find  important  correlations  of  absolute  magnitude 
with  color  and  with  the  velocity  of  motion  through  space ;  and 
finally,  we  learn  that  the  stellar  system  represents  a  high 
degree  of  organization  in  its  motions,  as  well  as  in  its  form 
and  structure. 


THE   100-INCH  REFLECTING  TELESCOPE  AT 
MOUNT  WILSON1 

In  September,  1906,  Director  George  E.  Hale  announced 
that  Mr.  John  D.  Hooker,  of  Los  Angeles,  had  presented  to 
the  Carnegie  Institution  of  Washington  the  sum  of  $45,000  to 
be  used  to  purchase  for  the  Solar  Observatory  a  disk  of  glass 
100  inches  (2.54  m.)  in  diameter  and  13  inches  (33  cm.)  thick 
and  to  meet  other  expenses  incident  to. the  construction  of. a 
100-inch  mirror  for  a  reflecting  telescope  of  50  feet  (15.24  m.) 
focal  length. 

Mr.  Hooker  had  been  interested  in  the  Solar  Observatory 
from  the  beginning.  In  1904  he  provided  the  funds  which  per- 
mitted Professor  Barnard  to  bring  the  Bruce  photographic 
telescope  from  the  Yerkes  Observatory  to  Mount  Wilson  and 
to  spend  the  period  from  December,  1904,  to  September,  1905, 
in  photographing  portions  of  the  Milky  Way  not  readily 
accessible  from  more  northern  stations.  These  photographs,2 
we  may  note  in  passing,  were  excellent,  and  fully  confirmed 
the  favorable  opinions  which  had  been  formed  of  the  suitability 
for  astronomical  work  of  the  atmospheric  conditions  on  Mount 
Wilson. 

In  his  deed  of  gift  Mr.  Hooker  specifically  left  the  Carnegie 
Institution  free  of  any  obligation  to  accept  the  mirror  or  to 
provide  a  mounting  for  it.  He  recognized  the  fact  that  the 

1  An    illustrated    lecture    describing   this    powerful    telescope    was    delivered    in 
San   Francisco    on   April    19,    1918,   by    Professor    G.    W.    Ritchey   to   conclude    the 
second  series  of  Adolfo  Stahl  Lectures  in  Astronomy.     Unfortunately,  it  has  been 
impossible   for   Mr.    Ritchey  to   put  this  lecture   into   written   form,   since   his   time 
has  been   completely   occupied   in   war  service  for  the   United    States    Government. 
It    was    therefore    decided,    after    consultation    with    members    of    the    staff    of    the 
Mount  Wilson  Observatory,  and  with  Mr.  Ritchey's  consent,  to  substitute  for   his 
address  the  present  paper  compiled  by  the  editor  of  this  volume. 

The  compilation  is  based  chiefly  upon  the  data  in  the  Annual  Reports  of  the 
Director  of  the  Solar  Observatory,  supplemented  by  data  kindly  supplied  by 
Mr.  F.  G.  Pease,  the  man  most  closely  associated  with  the  design  and  construction 
of  the  mounting  of  the  great  telescope.  Other  published  statements  have  also  been 
used,  and  particularly  the  abstract  of  Professor  Hale's  recent  address  to  the  Royal 
Astronomical  Society  printed  in  the  December,  1918,  number  of  The  Observatory. 
It  has  seemed  unnecessary,  in  general,  to  use  quotation  marks  in  a  paper  which 
consists  almost  entirely  of  direct  and  indirect  quotations.  R.  G.  A. 

2  See  Plate  XLIX  for  a  reproduction  of  one  of  these  photographs.     Many  of 
the  finer  details  shown  on  the  original  negative  are   of  course  lost  in  the  process 
of  reproduction. 


PLATE  L.     THE  ICO-lNCH  MIRROR. 
(In  the  optical  testing  room,   Pasadena.) 


THE  100-INCH  REFLECTING  TELESCOPE  247 

construction  of  a  reflector  of  such  great  dimensions  must  be 
regarded  as  an  experiment.  It  involved,  in  the  first  place,  the 
casting  of  a  block  of  glass  of  sufficient  homogeneity  weighing 
4l/2  tons  (the  disk  of  the  60-inch  reflector,  then  the  largest 
silver-on-glass  reflector  in  the  world,  weighs  one  ton ! ) .  Grant- 
ing that  this  could  be  accomplished,  and  that  it  could  be  con- 
verted into  a  satisfactory  mirror  and  provided  with  a  mount- 
ing capable  of  carrying  it  with  the  necessary  precision,  it  would 
still  remain  a  question  whether  the  atmospheric  conditions  on 
Mount  Wilson  or  at  any  other  station  would  prove  sufficiently 
good  to  permit  so  great  an  aperture  to  be  used  to  full  advan- 
tage. While  he  was  fully  aware  of  these  facts  and  did  not  un- 
derestimate the  magnitude  of  the  obstacles  that  must  be  over- 
come, he  perceived  and  appreciated,  with  the  understanding 
of  one  who  had  himself  invented  and  developed  mechanical 
appliances,  that  experiment  was  necessary  to  progress,  and  he 
did  not  hesitate  to  provide  the  means  for  undertaking  an  opti- 
cal experiment  on  so  large  a  scale. 

He  had  evidently  considered  the  matter  very  carefully 
before  making  his  gift.  He  knew  the  reputation  and  the  past 
performances  of  the  French  Plate  Glass  Companies  of  St. 
Gobain  which  had  cast  the  block  of  glass  for  the  60-inch  mir- 
ror; he  had  absolute  confidence  in  the  ability  of  Mr.  Ritchey 
to  make  an  essentially  perfect  mirror  100  inches  in  diameter; 
he  did  not  question  the  power  of  engineers  to  design  and  build 
an  adequate  mounting;  and  he  had  a  strong  desire  to  realize 
the  great  possibilities  in  astrophysical  research  which  such  a 
large  reflector  would  open.  Even  if  it  should  prove  that  the 
great  telescope  could  be  utilized  to  the  fullest  advantage  on 
only  a  very  few  nights  in  the  year,  its  construction  would  still 
be  desirable ;  and  it  had  already  been  shown  that  the  conditions 
on  Mount  Wilson  were  good  enough  on  a  large  percentage  of 
nights  in  the  year  to  promise  results  fully  commensurate  with 
the  size  of  the  mirror  in  several  classes  of  astronomical  work 
in  which  large  light-gathering  power  rather  than  the  most  per- 
fect definition  is  essential — as,  for  example,  the  measurement 
of  the  heat  radiation  of  the  stars,  or  the  spectroscopic  study  of 
very  faint  stars  and  spiral  nebulae. 

In  announcing  this  gift  Mr.  Hale  said :    "No  provision  has 


248  THE  ADOLFO  STAHL  LECTURES 

yet  been  made  for  the  mounting  and  dome.  It  is  not  known 
from  what  source  funds  for  this  purpose  will  come,  but  I  be- 
lieve a  donor  will  be  found  by  the  time  they  are  needed."  This 
faith  was  justified  by  the  event.  Mr.  Hale,  in  view  of  the  very 
generous  support  it  was  already  affording  the  Solar  Observa- 
tory, had  not  intended  to  ask  the  Carnegie  Institution  for  funds 
for  the  mounting  and  dome;  but  the  splendid  results  obtained 
with  the  60-inch  and  the  greater  possibilities  of  the  100-inch 
appealed  so  strongly  to  Mr.  Andrew  Carnegie  that  when  mak- 
ing a  new  gift  of  ten  million  dollars  to  the  Carnegie  Institu- 
tion, in  1912,  he  specified  that  provision  should  be  made  from 
this  grant  both  for  the  mounting  and  for  the  dome. 

In  September,  1906,  the  4^ -ton  block  of  glass  was  ordered 
from  the  French  Plate  Glass  Companies  of  St.  Gobain.  The 
largest  glass-melting  pots  they,  then  had  held  only  \y2  tons, 
hence  it  was  necessary  to  make  three  pourings  and  to  provide 
special  appliances  to  combine  these  into  a  single  block  and 
to  extend  the  time  of  annealing  over  a  long  period  to  reduce 
the  danger  of  internal  strain  arising  from  cooling.  By  June. 

1907,  everything  was  in  readiness  and  the  first  attempt  was 
made  early  in  July.     It  was  not  successful  and  repeated  trials 
were  necessary  before  a  block  was  obtained  which  the  firm 
considered  suitable.     This  disk  arrived  in  Pasadena  late  in 

1908.  Notwithstanding  the  pains  taken  in  the  casting,  it  was 
found  at  the  first  inspection  by  the  opticians  that  there  were 
large  sheets  of  bubbles  in  the  glass,  due  to  the  three  separate 
pourings,  and  the  disk  was  immediately  rejected. 

Though  the  work  was  trying  and  expensive,  the  glass  com- 
pany at  once  cheerfully  proceeded  to  further  experiments. 
They  built  a  furnace  in  which  twenty  tons  of  glass  could  be 
melted  at  one  time,  and  in  1910  and  again  in  1911  succeeded 
in  making  disks  of  the  requisite  size  at  a  single  pouring,  but 
unfortunately  on  each  occasion  these  cracked  in  the  annealing. 
This  long  delay  led  to  further  examination  of  the  disk  already 
at  Pasadena,  and  it  was  found  that  the  sheets  of  air  bubbles 
did  not  approach  the  surface  so  closely  as  to  interfere  with 
securing  a  perfect  paraboloidal  figure.  Tests  also  showed  that 
the  glass  as  a  whole  was  firmly  knitted  together  in  spite  of  the 
presence  of  the  bubbles ;  the  only  obstacle  to  its  successful  use 


THE  100-INCH  REFLECTING  TELESCOPE  249 

as  an  astronomical  mirror,  therefore,  would  be  the  existence 
of  strains  in  the  glass  that  would  prevent  it  from  maintaining 
its  figure  under  changes  of  temperature.  Whether  or  not  such 
strains  were  present  could  only  be  determined  by  testing  the 
glass  under  a  greater  range  of  temperature  than  that  between 
the  maximum  to  be  expected  in  the  dome  (in  summer)  and 
the  minimum  (in  winter). 

While  the  experiments  of  the  glass-  company  were  in 
progress,  preparations  were  being  made  in  Pasadena  for  the 
work  of  the  opticians  in  figuring  the  mirror.  These  in  them- 
selves involved  careful  planning  and  not  a  little  work.  There 
was  erected  in  Pasadena,  in  1906-07,  a  special  building  (the 
Hooker  Building)  which  included  a  fire-and-earthquake-proof 
room,  34  by  20  feet,  in  which  to  figure  the  glass,  and,  opening 
from  it,  a  testing  hall  100  feet  long  and  10  feet  wide,  both  of 
which  could  be  kept  at  constant  temperature.  The  air  enter- 
ing the  workroom  was  filtered,  the  walls  of  the  room  were 
varnished  with  shellac,  and  the  floor  was  kept  wet  to  prevent 
dust  from  rising  and  producing  scratches  on  the  glass.  Since 
no  mirror  even  approximating  this  one  in  size  had  ever  been 
figured,  it  was  necessary  to  design  and  construct  a  special 
grinding  machine,  special  grinding  and  polishing  tools,  and 
other  apparatus.  A  60-inch  disk  for  a  plane  mirror  (optically 
plane)  was  received  in  1909  and  work  on  figuring  and  polish- 
ing it,  in  itself  a  problem  of  no  small  magnitude,  was  begun. 

All  of  this  preliminary  work  was  sufficiently  advanced  to 
permit  of  making  the  temperature  tests  on  the  great  disk  in 
the  course  of  the  year  1911.  The  disk,  for  this  purpose,  was 
ground  to  a  rough  spherical  surface ;  this  figure  was  examined 
after  the  temperature  had  been  reduced  to  45°  F.  and  main- 
tained at  that  point  for  several  days.  As  no  distortion  in  figure 
could  be  observed,  the  temperature  was  next  raised  to  92°  F. 
and  the  tests  repeated.  These  also  showed  no  distortion  of 
figure  and  it  thus  seemed  safe  to  proceed  on  the  assumption 
that  no  prohibitive  strains  existed  in  the  glass. 

The  work  of  figuring  a  great  lens  or  mirror  cannot  be  hur- 
ried. The  grinding  must  be  done  with  the  greatest  care,  and 
frequent  tests  must  be  made  to  determine  the  precise  stage 
reached ;  the  farther  the  work  proceeds  the  more  often  the 


250  THE  ADOLFO  STAHL  LECTURES 

tests  must  be  made.  During  the  three  months  of  the  year 
when  it  was  necessary  to  employ  artificial  heat  in  the  work- 
room in  Pasadena  it  was  found  difficult  to  maintain  satisfac- 
tory temperature  conditions,  and  these  months  were  accord- 
ingly devoted  to  the  preliminary  work  on  the  subsidiary  optical 
parts  rather  than  to  furthering  the  figuring  of  the  main  mir- 
ror. The  plan  of  figuring  adopted  by  Mr.  Ritchey  involved 
bringing  the  mirror  first  to  a  perfect  spherical  surface  with  a 
radius  of  curvature  of  about  84  feet,  and  then  making  the  rela- 
tively small  corrections  needed  to  convert  this  surface  into  that 
of  a  paraboloid. 

As  an  illustration  of  the  minor  problems  that  had  to  be 
solved,  it  may  be  noted  that  when  the  mirror  began  to  approach 
the  perfect  spherical  surface  demanded  it  was  found  that, 
although  fans  had  been  installed  to  produce  a  thorough  mix- 
ture of  the  air  in  the  optical  room  and  testing  hall,  sufficient 
stratification  still  existed  to  affect  the  tests  seriously,  and  suf- 
ficient temperature  variation  between  the  top  and  bottom  of 
the  glass,  when  the  mirror  was  set  upright  on  its  edge,  to 
introduce  a  small  amount  of  distortion.  Special  devices  had 
to  be  employed  to  overcome  these  conditions. 

Before  the  close  of  the  year  1914  a  satisfactory  spherical 
surface  had  been  obtained  and  the  work  of  parabolizing  was 
begun.  By  September,  1915,  80  per  cent  of  the  total  change 
necessary  had  been  accomplished,  involving  90  days  of  actual 
figuring  with  the  large  machine.  Optical  tests  were  made  each 
morning  after  a  day's  figuring;  frequently  repeated  tests  on 
different  days  were  necessary  before  figuring  could  be  re- 
sumed. These  tests  were  made  both  at  the  center  of  curva- 
ture and  at  the  focus  of  the  paraboloid;  the  former  method 
is  better  for  determining  the  figure  of  the  mirror  as  a  whole, 
while  the  latter  test  is  invaluable  for  detecting  and  correcting 
zonal  errors  in  the  general  curvature.  Throughout  the  figuring 
the  Hartmann  method  of  testing  was  used,  the  measurements 
of  the  photographic  plates  by  Mr.  Adams  furnishing  the  most 
explicit  data.  Thus,  through  alternate  figuring  and  testing,  the 
mirror  was  skilfully  brought  to  a  perfect  optical  surface  early 
in  the  year  1916.  Members  of  the  Astronomical  Society  of  the 
Pacific  who  visited  Pasadena  in  August,  1916,  have  vivid 


PLATE  LI. 

A  tube-section  of  the  100-inch  telescope  on  the  road  up  Mount  Wilson. 

(See  p.  252.) 


THE  100-INCH  REFLECTING  TELESCOPE  251 

memories  of  the  great  glass  standing  in  the  optical  shop  as 
shown  in  our  illustration.  Some  idea  of  the  precision  with 
which  the  figuring  and  testing  had  to  be  done  may  be  gained 
from  the  statement  that  at  the  center  of  the  mirror,  where  the 
difference  is  greatest,  the  depth  of  the  finished  paraboloid  dif- 
fers from  that  of  the  nearest  spherical  surface  (to  which  the 
glass  was  brought  in  preparation  for  parabolizing)  by  almost 
exactly  0.001  inch  (0.025  mm.)  ! 

The  following  numerical  data  may  be  of  interest:  When 
Mr.  Hooker's  gift  was  first  announced  in  1916  it  was  the  inten- 
tion, as  stated  in  the  first  paragraph  of  this  paper,  to  make  the 
focal  length  50  feet,  giving  a  ratio  of  focal  length  to  aperture 
of  6:1.  Later  it  was  decided  to  adopt  a  smaller  ratio,  approxi- 
mately 5:1,  and  the  actual  focal  length  of  the  finished  mirror 
is  found  to  be  507.5  inches,  the  clear  aperture  being  100.4 
inches.  The  depth  of  the  curve  at  the  center  of  the  mirror  is 
about  1.25  inches;  the  thickness  of  the  glass  at  the  edge,  12.75 
inches;  the  weight  is  nearly  9,000  pounds.  A  curvature  of 
only  1.25  inches  in  a  diameter  of  100.4  sounds  small,  but  the 
concavity  thus  formed  will  hold  35  gallons  of  water. 

Notwithstanding  the  great  size  and  weight  of  the  glass,  the 
work  of  silvering  its  surface  was  accomplished  without  diffi- 
culty by  placing  the  mirror  upon  the  large  polishing  machine, 
which  permitted  rocking  it  during  the  operation  and  tipping 
it  to  pour  off  the  solutions,  both  operations  being  accomplished 
by  the  motor-driven  mechanism.  The  silver  surface  was  pro- 
duced by  the  approved  modern  method  of  pouring  upon  the 
glass  simultaneously  a  dilute  silver  solution  and  a  dilute  reduc- 
ing solution,  thus  forming,  if  skilfully  done,  a  deposit  of  pure 
silver  of  uniform  density  over  the  entire  disk.  Thirty-two 
ounces  of  silver  nitrate  were  used,  and  it  required  15  minutes' 
time  to  form  a  coat  of  satisfactory  density.  This  silver  film, 
after  being  washed  carefully  with  distilled  water  and  allowed 
to  dry,  was  burnished  with  the  large  polishing  machine  and  a 
cushioned  tool  34  inches  in  diameter,  covered  with  six  selected 
chamois  skins. 

Early  in  July,  1917,  the  great  mirror  was  transported  to 
the  observatory  prepared  for  it  on  Mount  Wilson.  The  mir- 
ror was  crated  in  a  strong  box  lined  with  building  paper  and 


252  THE  ADOLFO  STAHL  LECTURES 

supported  on  its  edge  by  a  heavy  framework  bolted  to  the  bed 
of  the  motor  truck.  To  reduce  the  amount  of  vibration,  nu- 
merous strong  springs  were  inserted  between  the  box  and  the 
framework.  The  top  of  the  mirror  box  when  placed  on  the 
truck  was  about  14  feet  from  the  ground,  and  its  weight, 
including  the  support,  was  7.5  tons.  Although  the  truck  used 
had  been  specially  designed  to  carry  the  heavy  castings  (one 
of  nearly  11  tons  weight)  up  the  steep  mountain  road  (the 
average  gradient  is  about  1  in  11),  and  the  road  had  been  wid- 
ened during  the  preceding  years  and  carefully  inspected  just 
before  the  trip,  one  can  readily  imagine  the  feelings  of  relief 
of  the  members  of  the  observatory  staff  when  the  mirror  was 
safely  at  the  summit.  One  of  our  illustrations  gives  a  vivid 
idea  of  what  might  possibly  have  happened.  In  carrying  a 
tube-section  up  the  mountain  a  soft  place  in  the  road  caused 
one  wheel  of  the  truck  to  drop  a  few  inches.  A  chance  tree  and 
quick  work  alone  prevented  a  disaster.  A  few  more  inches  and 
truck  and  all  might  have  rolled  hundreds  of  feet  down  into  a 
canyon.  A  "movie-man"  accompanied  the  truck  on  nearly  all 
trips  when  heavy  pieces  of  the  mounting  were  carried  up  to  the 
observatory. 

Plans  for  mounting  the  great  mirror  had  engaged  the 
attention  of  Director  Hale,  Professor  Ritchey  and  other  mem- 
bers of  the  staff  from  an  early  date.  The  great  weight  of  the 
moving  parts  of  the  telescope  had  to  be  considered  as  well  as 
the  adaptability  of  the  mounting  to  the  various  programs  of 
work  it  was  hoped  might  be  undertaken.  The  designs  of 
mounting  and  dome  finally  adopted  are,  as  Mr.  Hale  puts  it, 
really  composite,  being  the  work  of  Professor  Ritchey  in  the 
earlier  stages  and  later  of  Mr.  F.  G.  Pease  (who  has  also 
supervised  their  erection)  ;  but  doubtless  they  incorporate  also 
many  suggestions  by  Mr.  Hale,  Mr.  Adams  and  others.  The 
mounting  is  of  what  is  known  as  the  English  type,  which  has 
the  advantage  of  compactness,  and,  in  view  of  the  great  weight 
(100  tons)  of  the  moving  parts,  is  also,  in  the  opinion  of  the 
designers,  safer  than  any  other  type.  By  way  of  comparison, 
it  may  be  noted  that  the  moving  parts  of  the  great  refractor  of 
the  Lick  Observatory  weigh  only  14^  tons.  The  English  type 
of  mounting,  however,  has  the  disadvantage  that  the  northern 
pier  prevents  the  telescope  from  being  turned  upon  a  small 


PLATE  LIT.     THE  100-INCH   REFLECTOR,   OCTOBER,   1917. 


THE  100-INCH  REFLECTING  TELESCOPE  253 

area  of  the  sky  centering  at  the  north  pole.  As  the  illustra- 
tion shows,  the  telescope  is  hung  within  a  yoke  or  double  fork, 
which  measures  32  feet  8  inches  by  16  feet  2  inches.  To  sup- 
port the  great  weight,  the  system  introduced  by  Dr.  Common 
was  used,  in  which  the  greater  part  of  the  weight  is  taken  up 
by  floating  the  polar  axis  in  mercury.  The  upper  float,  on  the 
northern  pier,  carries  about  40  tons  weight,  the  lower  about  60 
tons.3 

The  instrument  is  mounted  upon  a  pier  of  reinforced  con- 
crete measuring  45  by  20  feet  at  the  ground  level  and  32  feet 
11  inches  in  height,  raising  the  center  of  motion  of  the  tele- 
scope to  a  distance  of  50  feet  above  the  ground.  The  top  of 
the  pier  consists  of  a  circular  concrete  floor  6  inches  thick  ( 18 
inches  over  the  pier  proper)  and  53  feet  10  inches  in  diameter. 
Massive  reinforced  concrete  brackets  extending  outward  from 
the  pier  on  the  east  and  west  sides  help  to  support  this  floor. 
The  pier  itself  is  hollow,  and  within  it  are  two  floors,  the  lower 
one  for  the  reservoir  for  the  large  mirror  temperature-control 
described  in  the  following  paragraph,  the  upper  one  for  the 
driving-clock,  worm,  and  quick-motion  right-ascension  mecha- 
nism. A  room  for  resilvering  the  mirror,  when  necessary,  is  also 
included,  and  an  electric  elevator  for  handling  the  mirror  moves 
through  a  14-foot  opening  near  the  center  of  the  pier. 

The  clock-work  driving- worm  goes  into  a  wheel  which  is  17 
feet  in  diameter.  This  wheel  was  cut  in  position  on  the  moun- 
tain, each  tooth  being  cut  separately  under  the  microscope,  the 
wheel  then  hobbed  and  finally  ground.  All  motions  of  the  tele- 
scope are  effected  and  controlled  by  electric  motors.  The  mir- 
ror itself  stands  in  its  cell  upon  the  usual  lever-support  system 
for  the  rear  support  and  upon  four  edge-arcs  resting  upon 
knife-edges  for  edge  supports.  Behind  the  supporting  plate 
there  is  a  flat  coil  of  copper  pipe  connected  in  series  witti  several 
turns,  one  above  the  other,  around  the  lower  edge  of  the  mirror. 
An  anti-freezing  solution  whose  temperature  may  be  automat- 
ically controlled  is  circulated  through  these  pipes  from  tanks  in 
the  pier.  Fans  blow  over  these  coils  and  circulate  the  air  all 


3  The  heavier  parts  of  the  mounting  were  made  at  the  Fore  River  Shipyards, 
near  Boston,  and  it  was  necessary  to  send  the  four  tube-sections  by  steamship  to 
Los  Angeles  Harbor,  for  they  were  too  large  for  railroad  clearances.  (The  tube 
is  11  feet  in  diameter.)  All  the  smaller  parts  and  accessories  were  made  in  the 
observatory  shops  at  Pasadena. 


254  THE  ADOLFO  STAHL  LECTURES 

around  the  mirror.  The  mirror,  supports,  lower  part  of  cell, 
coils,  etc.,  are  all  enclosed  in  a  cork-board  chamber  built  in- 
tegral with  the  telescope,  the  cover  above  the  mirror  being 
opened  in  the  form  of  eight  sectors  when  observations  are 
made. 

It  is  planned  to  use  the  mirror  at  the  primary  focus  (507.5 
inches)  for  a  large  proportion  of  the  work;  but  two  convex 
(hyperbolic)  secondary  mirrors  are  also  provided  to  permit 
the  use  of  the  telescope  in  the  Cassegrain  form.  One  of  these 
(28.75  inches  in  diameter  and  over  6.5  inches  thick)  gives, 
with  the  main  mirror,  an  equivalent  focal  length  of  1,606 
inches;  the  other  (25  inches  in  diameter  and  5.5  inches  thick), 
an  equivalent  focal  length  of  approximately  3,011  inches. 
The  tube-sections  holding  these  smaller  upper  mirrors  can  be 
put  into  position  with  the  aid  of  a  crane  attached  to  the  dome 
and  moving  with  it. 

The  mounting  is  so  constructed  that  the  telescope  can  be 
used  also  in  the  coude  form,  the  light  gathered  by  the  large 
mirror  being  thrown  down  the  polar  axis  to  the  south  by  a 
secondary  mirror.  The  pier  proper  has  an  extension  running 
out  to  the  south  under  the  dome,  the  top  sloping  at  an  angle 
corresponding  to  the  latitude  of  the  observatory.  A  powerful 
concave-grating  or  plane-grating  spectrograph  rigidly  attached 
to  this  extension,  which  is  enclosed  by  an  outer  concrete  wall 
and  roof,  will  make  it  possible  to  secure  spectra  of  the  brighter 
stars  on  a  very  large  scale,  with  an  equivalent  focal  length  of 
the  telescope  of  250  feet — a  project  which  Mr.  Hale  has  long 
cherished. 

The  circular  steel  building,  100  feet  in  diameter,  which 
shelters  the  great  telescope  is  of  simple,  almost  austere,  de- 
sign, bu,t  it  is  fully  worthy  of  the  instrument  within.  The  out- 
standing impression  it  makes  upon  the  beholder  is  one  of  mass- 
ive dignity.  Resting  upon  two  concentric  rings  of  concrete 
piers  as  a  foundation,  it  is  as  nearly  as  possible  fire-and-earth- 
quake-proof.  The  walls  are  double  and  the  dome  above  is 
double-sheathed  for  protection  against  the  Sun.  The  upper 
part  of  the  dome,  weighing  494  tons,  rotates  on  rails.  The 
upper  floor,  around  the  circular  top  of  the  great  pier,  forms 
part  of  the  dome  and  rotates  with  it.  This  serves  to  stiffen 


PLATE  LIII.    DOME  OF  THE  100-INCH  TELESCOPE,  MOUNT  WILSON. 


As  seen   from  the  top  of  Jhe   150-foot  tower.     Back  range  of  the   San 
Gabriel  Mountains  in  the  distance. 


THE  100-INCH  REFLECTING  TELESCOPE  255 

the  dome  and  to  enable  it  to  turn  very  quickly  and  with  little 
vibration.  Electric  motors  turn  the  dome,  operate  the  shut- 
ters, wind-screen,  etc.,  while  others  are  used  to  manipulate  the 
telescope.  In  all,  35  motors  are  involved,  and  the  electric  wir- 
ing proved  a  task  of  very  considerable  difficulty. 

In  November,  1917,  it  was  possible  to  make  the  preliminary 
tests  of  the  100-inch  reflector  under  fair  conditions  of  seeing. 
The  instrument  was  not  yet  fully  adjusted  and  the  mirror  tem- 
perature-control was  not  working.  Nevertheless,  the  Moon 
and  Saturn  showed  an  extraordinary  amount  of  detail ;  the  star 
images,  however,  showed  multiple,  with  considerable  flare. 
War  work  was  absorbing  the  energies  and  time  of  the  staff  to 
such  an  extent  that  the  second  test  could  not  be  made  until 
September,  1918.  Then,  with  the  mirrors  carefully  lined  up 
(a  compression  ring  having  been  added  to  keep  the  convex 
mirror  in  position)  and  with  the  mirror  temperature-control 
system  in  operation,  it  was  found  that  the  multiple  images  and 
all  flare  had  disappeared  and  that  the  star  images,  in  Mr.  Eller- 
man's  phrase,  were  "as  hard  and  fine"  as  those  seen  in  any  tele- 
scope. The  spectrograph  for  the  Cassegrain  focus  at  1,606 
inches  has  been  completed  except  for  the  prism  temperature- 
control,  and  on  December  23,  1918,  Mr.  Pease  secured  several 
spectrograms  with  it.  The  ease  with  which  the  telescope 
worked  he  found  to  be  remarkable;  and  the  driving  clock, 
which  is  designed  to  carry  a  maximum  driving  weight  of  two 
tons,  ran  with  great  surplus  power  with  a  weight  of  only  1,400 
pounds,  carrying  the  telescope  from  a  position  four  hours  east 
of  the  meridian  to  one  four  hours  west  without  the  slightest 
difficulty.  Minor  corrections  and  improvements  still  remain 
to  be  made  to  bring  the  instrument  into  the  perfect  adjustment 
at  which  its  designers  aim.  For  example,  the  polar  axis  will 
be  more  accurately  aligned  and  a  minute  periodic  error,  which 
at  present  gives  a  drift  of  one-tenth  of  a  second  of  arc,  will 
be  eliminated  from  the  driving-clock.  But  it  is  safe  to  say  that 
the  telescope  has  now  fairly  passed  the  experimental  stage  and 
that  it  will  be  in  use  on  a  regular  program  of  observation  in 
the  early  spring  of  1919. 

What  will  it  do  that  smaller  telescopes  can  not  do  ?    This  is 
the    question    that    interests    astronomers    and    laymen    alike. 


256  THE  ADOLFO  STAHL  LECTURES 

Prophecy  is  always  dangerous,  and  Professor  Hale  and  his 
associates  are  wisely  reticent  as  to  the  answer.  But  a  genera) 
statement  may  be  ventured  upon  here. 

First  of  all,  it  must  be  said  most  emphatically  that  sensa- 
tional "discoveries"  are  not  to  be  expected.  Readers  of  Dr. 
Curtis's  lecture  on  "Astronomical  Discovery,"  in  this  volume, 
will  hardly  need  to  be  reminded  of  this  fact.  Unexpected  dis- 
coveries, in  the  popular  sense,  may  come,  of  course,  but  ad- 
vances in  astronomy  at  the  present  day  are  made  chiefly  by  the 
analysis  of  great  quantities  of  material  accumulated  by  patient 
and  persistent  observation,  along  lines  laid  down  in  carefully 
matured  plans ;  and  it  is  in  work  of  this  character  that  the  new 
telescope  will  unquestionably  be  employed.  Here  it  possesses 
two  important  advantages  over  other  telescopes,  arising  from 
its  great  aperture,  namely,  the  increase  in  light-gathering 
power  and  the  increase  in  resolving  power.  The  former  brings 
within  its  grasp  fainter  stars;  the  latter  makes  it  possible  to 
study  more  minutely,  photographically  as  well  as  visually,  the 
details  of  various  classes  of  celestial  objects,  such  as  the  nebu- 
lae, for  example. 

The  theoretical  increase  in  light-gathering  power  of  similar 
telescopes  varies  as  the  square  of  the  aperture,  and  this  increase 
in  the  case  of  the  100-inch  with  respect  to  the  60-inch  will  hold 
both  at  the  primary  focus  and  at  the  Cassegrain  and  coude  foci, 
for  the  proportion  of  light  cut  out  by  the  secondary  mirrors  is 
about  the  same  in  the  two  instruments.  Speaking  in  general 
terms,  it  ma^  be  said  that  the  60-inch  reflector  records  the  pho- 
tographic images  of  stars  as  faint  as  the  twentieth  magnitude 
and  gives,  with  reasonable  exposure  times,  spectrograms  of  stars 
about  6.5  magnitude,  which  are  comparable  with  those  of  stars 
of  5.5  magnitude  taken  with  the  Mills  spectrograph  attached 
to  the  36-inch  refractor  of  the  Lick  Observatory.  The  100-inch 
should  be  able  to  proceed,  in  both  cases,  to  stars  about  one  mag- 
nitude fainter.  Stated  thus,  the  gains  may  seem  unimportant ; 
but  turn  to  Table  I,  page  214,  in  the  lecture  by  Professor  Scares 
in  this  volume.  It  will  then  be  seen  that  the  gain  of  a  single 
magnitude  brings  many  tens  of  millions  of  fainter  stars  within 
the  range  of  photographic  records  and  fully  triples  the  number 
of  brighter  stars  available  for  spectrographic  studies.  These 


THE  100-INCH  REFLECTING  TELESCOPE  257 

gains  will  be  of  the  highest  consequence  in  the  solution  of  some 
of  the  fundamental  problems  of  stellar  motions  and  of  stellar 
distribution  in  space ;  and  experience  with  the  60-inch  reflector 
indicates  that  the  atmospheric  conditions  on  Mount  Wilson  are 
amply  good  enough  to  permit  their  realization. 

Whether  or  not  the  full  power  of  the  telescope  can  be 
realized  in  practice  in  studies  requiring  fine  definition — as,  for 
example,  in  the  study  of  minute  details  of  planetary  surfaces  or 
of  the  nebulae,  or  in  the  securing  of  stellar  spectra  of  very 
high  dispersion  and  resolution — it  is  impossible  to  predict, 
whatever  our  hopes  and  even  expectations  may  be.  We  know 
that  an  aperture  of  36  inches  relatively  to  one  of  6  inches 
magnifies  the  atmospheric  disturbances  almost  in  proportion 
to  the  gain  in  aperture.  It  is  not  certain  that  the  same  law 
will  hold  when  we  proceed  from  a  36-inch  aperture  to  one  of 
100  inches.  There  are  some  indications  from  the  experience 
with  the  60-inch  and  with  the  new  72-inch  (at  the  Dominion 
Astrophysical  Observatory)  that  it  will  not.  But  even  should 
this  prove  to  be  the  case,  there  will  be  occasional  nights  when 
the  conditions  will  be  favorable  to  work  requiring  the  finest 
definition,  and  we  may  rest  assured  that  the  fullest  advantage 
will  be  taken  of  every  such  opportunity. 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 
BERKELEY 

Return  to  desk  from  which  borrowed. 
This  book  is  DUE  on  the  last  date  stamped  below. 


ASTRONOMY   LIEiRARY 


JUL  25  1972 


LD  21-100m-ll,'49(B7146sl6)476 


104 1 92 


462946 


UNIVERSITY  OF  CAUFpRNIA  UBRARY 


